I'd like to note that, as with any project, this is a work in progress. I've revised some of my opinions and stances on certain powers and how they ought to be slotted, though these are ones that were already heavily influenced by my opinion. Experiment! You may find that a different build strategy works better for you. If that's the case, more power to ya!
Here are the deviations from the above guide that I currently run on my controller:
Slot[05] Level 6 : Snow Storm /EndRdx,Slow,Slow
Slot[09] Level 14 : Fly /Fly,Fly,Fly
I moved two slots from Fly over to Snow Storm, as you can see, and made a liar out of myself by putting Slow enhs in them. Fly hits a speed cap at around 4 SOs, and I find 3 to be enough for my own use after all.
As for Snow Storm, sometimes mobs just don't want to stay rooted, and for this reason stacking SS and FR helps keep them from moving around too much while you get the rest off. It's far from perfect, but I found myself not using Snow Storm very often when it was 1 slotted, and retweaked to correct that. It also helps cut down on "runners" which is something everyone appreciates.
This is a wonderful guide you have put together; the rundown of each power chosen at each level, how to slot them and why is the sort of thing that distinguishes this as insightful and helpful. The remarks on alternative slotting/powers is gravy. Well done! 
A couple of questions...The Steamy Mist/Super Speed combo seems to be one of the better aspects of the build, so why not take advantage of the tactics it allows as soon as possible, maybe even in your teens? You would have to move Hasten up as well, but it seems like it would be worth it. Also, Hurricane seems to be really far down as well and some people contend that it is one of, if not the best power available in the strom summoning line so why not take it much earlier as well? Maybe push Hover/Fly and Manuevers/Tactics farther down to accommodate them? I may be looking at this a little too much from a (kinetics) defender's point-of-view, but that is why I am asking.
Again, this is one of the best guides I have read. The in-depth analysis is not just filler unlike many of the guides you run across. Great Job! 
Teflon man you forgot to mention one use for wormhole... Fishing!
D'OH! How could I forget Fishing!?! RIKTI IN THE WALLS! 
Re: SisBoomBah's questions. Well, it's ideas like these that prompted me to include the "Experiment!" note. The build is fairly modular - you can shift around the order in which you choose powers pretty easily without hurting yourself, and in the end you'll still have it all. I don't use Hurricane nearly as much as many people do, so I placed it farther down the build. If you'd rather make it a part of your early strategy, then by all means do so! Similarly, if you'd rather have the Mist/SS stealth combo going early, it's not hard to do. Pretty much the only ones that I abosolutely wouldn't change are the pre-10 powers, GDF, Singularity, and Freezing Rain. The rest are presented partly in order of importance, partly in order of preference, but primarily because it suits my preferred style of play. Yours is bound to be at least a little different, to tweaking it to suit yourself is not only possible, it's a good idea.
This is a guide, not a prison, and is only intended to present ideas. So long as you're planning ahead, customize it to your heart's content.
It occurs to me that I haven't weighed in on one aspect of the Gravity/Storm combination: the powers I haven't recommended. I suppose the fact that I have avoided them in this build is itself an implicit recommendation NOT to take them, but perhaps I ought to be clearer as to why.
- Crush. I did touch on this one briefly, but didn't cover everything. As a control power, this isn't so hot, but that doesn't leave it completely useless. Parsing experiments have shown that it is actually one of, if not the, best powers available to a Grav controller who wants a power to devote to damage dealing, to help with soloing (missions and the like); 6-slotting it with damage has fairly decent returns and doesn't put an undue strain on your Endurance bar. If you're interested in adding some damage dealing capabilities to your repertoire and don't want to burn a Power Pool on it, Crush is a good choice.
- Propel. The irony here is that all the resons why you might want to take Crush for damage don't apply to the power that's really ONLY good for that. Propel has a high End cost, very slow execution time, and damage that scales abysmally as your level climbs. It's a single-target nuke that sucks at nuking. The best reason to not get it is simply that it's supposed to do one thing well, but doesn't. This really is a tragedy, since it's hands-down one of the coolest powers in the set, and is often the reason many people are attracted to Gravity in the first place. I mean, you get to throw cars and forklifts! How cool is that? Sadly, the reality is truly awful.
- Dimension Shift. There are those who defend this power, but frankly I don't understand why. It's a panic button that doesn't always work (i.e. it has an accuracy check, and thus, it is possible to miss with it) and which generally will not see use more than once in a blue moon. That's not to say that it is completely useless - it's not - but that the ability to render a group of minions and lieuts immobile and unable to be affected by you or your team will hardly EVER be one you actually want to use. Ever been frustrated about that Fake Nemesis Personal Force Field? Yeah? Well that's what this does to every single opponent, only Dim Shift has the added bonus of holding them still so that you and your enemies can stare at each other, and nothing else. Until it wears off, and they shoot you in unison and you die. This power is #1 on the "The Devs need to change it if it's ever to be useful" list. Moment of Glory has more utility, and that's sad.
- Tornado. This power creates a pet tornado that does very minor damage and draws a lot of aggro - aggro that it doesn't keep, instead transferring the wrath of those PO'd mobs right to your fragile Controller skull. Not only that, it has a tendency to cause a fear effect in the mobs it attacks, prompting them to run away. It then picks a runner at random and follows him, most likely wandering near another group of mobs and aggroing THEM on you as well. There's a reason why nearly no one ever takes this power. It creates total chaos, and does nothing to mitigate the danger.
I actually went a completely different derection than this because I wanted to be able to solo if I wanted. This configuration allows me to solo a white and yellow lt pair(something my fire/emp cannot do).
I have:
propel - 1 acc/5 dmg(does about 45 dmg)..slow
lift - 1 acc/2 dmg(does about 20 dmg)..decently fast
gravity disortion - 1 acc/4 holds
cruch - 1 acc/ 1 timer
gale, crushing field, hover, and steamy mist are unslotted, fly is 3 slotted.
I am only 15 though, so anything can change......swift, health, stamina, gravity distortion field, freezing rain, hasten, O2 boost, thunderclap, and singularity will be chosen in that order....(i will also be adding acc to all powers)
Also, my friend has a 48 grav/storm and it is extremely soloable with large mobs, with no leadership powers at all. He uses thunderstorm all the time, if mobs are tight enough, it can hit multiple targets every strike.
....just my take)
Just my observations. Ever run into a Storm guy from the Patheon? Their Lightning Storm has a very annoying effect. It drains a lot of end with each hit. Since you are a controller, and a mob without end can't attack, he is therefore controlled. Also, since mob pooling has become faux paux, hurricane has become a little more useful. Gale to push the mobs into some convenient corner, then hurricane to keep them bunched up while you aoe crush and aoe grav and aoe lightning storm and aoe rain, etc etc I think we get the point.
First, here's another more recent Grav guide. It's good and a different take on our powers: Grav/Storm guide
Next a few things for you:
- Wormhole is becomming AoE in I4 and having it's recharge time increased.
- Propel is having it's damage increased in I4
- TP friend can be used to bring your sings from other floors or across the map to you. Slot a Range SO in it because sometimes 1 floor of a building is very far away from the other on the maps. Still doesn't always work, but maybe 90% of the time it does.
- Crush field works well with Freeze Rain. You get almost no hits on you and it keeps the mobs in the rain (chain CF just after FR, not before). This can cover your lack of a 2nd powerful hold (though with AoE wormhole coming any day now it might not matter as much, we'll see).
- With ancillary pools availabel I'd suggest Power or Ice. Hibernate is nice for avoiding aggro while your sings and Lightning Storm do their work, and Power is fairly obvious in it's uses I think
- Thunder Clap is auto hit at the moment but it's magnitude (1) only affects Minions, so don't bother with the ACC enhances, and don't expect it to disorient LTs and above. This may combo well with Wormhole soon though because both have disorients. So I say slot some Disorient Dur enhances in it and call it a day. (they may change the auto-hit at some point, but doesn't appear to be have changed in I4)
I know this guide is older, but I offer a note to edit for the future on O2 Boost:
[ QUOTE ]
Its the only Heal in the Controller secondaries that is ranged single-target, which means that its easier to heal the allies who need it, but only one at a time.
[/ QUOTE ]
Untrue. Heal Other from the /Empathy set is also a single-target ranged heal.
Also useful to note post-I6 is that with Containment, Propel is now a much more attractive power, as you get double damage if you've held your target before using it.
The only thing I have to add is that Grav/Storm seems to be the preferred controller of Statesman. So that makes it cool in itself.
Arc Salvo: Okay hold one sec guys, we can't just rush in blindly vs these Nemesis, they've got these ranged aoe's tha-
Teammate1(charging in): Shut up, Arc Salvo, you lame*$% Viewtiful Joe wannabe! What do you know?!
Teammate2(also charging): yeah, ST#& arc salvo u PWR RANGR U!
Arc Salvo: *sigh*
It's fricken powerful.
Just as a general note for those reading this guide : the value for WormHole, particularly combined with Thunder Clap and Hurricane, has raised exponentially. It's not possible to control entire spawns constantly with just those powers.
This guide will be to my main build, Gravity Control/Storm Summoning. It is one of the best builds available for a controller who wants to focus almost purely on crowd control and doesnt mind giving up the soloability of Illusion or Fire (both of which took a hit recently). Note that Im writing this guide from a position where I can look back and see the mistakes that I made as well as the good choices this guide is not to what I DID, but to what I think you SHOULD do.
All statements made herein are just my own personal opinions. If you take offense to someone airing their opinions, please stop reading now.
Gravity / Storm build:
Slot[01] Level 1 : Lift /Acc
Slot[02] Level 1 : Gale /Acc
Slot[03] Level 2 : Gravity Distortion /Acc,EndRdx,Hold,Hold,Hold,Rchg
Slot[04] Level 4 : O2 Boost /EndRdx,Heal,Heal,Heal,Heal,Heal
Slot[05] Level 6 : Snow Storm /EndRdx
Slot[06] Level 8 : Crushing Field /Acc,EndRdx,Immob,Immob
Slot[07] Level 10 : Hover /DefBuf
Slot[08] Level 12 : Swift /Run
Slot[09] Level 14 : Fly /Fly,Fly,Fly,Fly,Fly
Slot[10] Level 16 : Freezing Rain /EndRdx,Slow,Slow,Rchg,Rchg,Rchg
Slot[11] Level 18 : Gravity Distortion Field /Acc,Hold,Hold,Hold,Rchg,Rchg
Slot[12] Level 20 : Health /Heal
Slot[13] Level 22 : Stamina /EndRec,EndRec,EndRec,EndRec,EndRec,EndRec
Slot[14] Level 24 : Maneuvers /EndRdx,DefBuf,DefBuf,DefBuf,DefBuf,DefBuf
Slot[15] Level 26 : Tactics /EndRdx,HitBuf,HitBuf,HitBuf,HitBuf,HitBuf
Slot[16] Level 28 : Hasten /Rchg,Rchg,Rchg,Rchg,Rchg,Rchg
Slot[17] Level 30 : Hurricane /HitDbf,HitDbf,HitDbf
Slot[18] Level 32 : Singularity /Dmg,Dmg,Rchg,Rchg,Rchg,Rchg
Slot[19] Level 35 : Thunder Clap /Acc,Dsrt,Dsrt,Dsrt
Slot[20] Level 38 : Steamy Mist /EndRdx,DefBuf,DefBuf,DefBuf,DefBuf,DefBuf
Slot[21] Level 41 : Super Speed /Run
Slot[22] Level 44 : Lightning Storm /EndRdx,Dmg,Dmg,Dmg,Dmg,Dmg
Slot[23] Level 47 : Wormhole /Acc
Slot[24] Level 49 : Vengeance /Heal
Gravity / Storm breakdown:
Slot[01] Level 1 : Lift /Acc
Your choices here are Crush or Lift. A single-target immobilize isnt bad, per se, but you wont use it much, if at all, in the later game. Lift has a quick execution time and guaranteed knockdown again most enemies, which makes it good as an interrupt useful if youre waiting for a hold to recharge. The damage is negligible, however, and the knockdown doesnt need slotting, so leave it alone.
Slot[02] Level 1 : Gale /Acc
Useful in the early game, especially indoors, for knocking a group of enemies off their feet. Im personally not a fan of it, since unless it hits every member of a mob you end up scattering the enemies, which makes AoEs more difficult. Youre stuck with it, though, so deal.
Slot[03] Level 2 : Gravity Distortion /Acc,EndRdx,Hold,Hold,Hold,Rchg
This is your bread-and-butter, so fully slot it. Its fairly accurate already, and with Tactics later on youll have even more, but this is a power youll never want to miss with, so go ahead and add one Accuracy. Three hold durations and a Recharge/PermaHasten will let you chain-cast it on bosses to lock them down, and an End Redux takes the edge off the damage that will do to your End bar, which will already be supporting toggles.
Slot[04] Level 4 : O2 Boost /EndRdx,Heal,Heal,Heal,Heal,Heal
Youre not a solo character, so you might as well get used to using team-friendly powers. O2 Boost lets you back up the main healer of a group or, in a pinch, allows you to step in and assume a hybrid control/healer function. Its the only Heal in the Controller secondaries that is ranged single-target, which means that its easier to heal the allies who need it, but only one at a time. Later in the game, it becomes very useful as a short-term protection against Disorient, which is golden in the stun-heavy late game.
Slot[05] Level 6 : Snow Storm /EndRdx
Very useful in the early game for slowing bosses and tough groups of enemies, which cuts down on incoming dps and makes the fight more manageable. Snow Storm puts enough of a drain on your End bar to make hard fights tricky if they go too long. I choose not to slot it with Slow enhances because they only affect the enemies movement speed, not Recharge rate, and my enemies are usually immobile already. The slots are better put to use elsewhere. Its worth noting that in the late game, high-level Archvillains seem to really hate Snow Storm and may very well kill you for it, even if youve done nothing else to draw aggro.
Slot[06] Level 8 : Crushing Field /Acc,EndRdx,Immob,Immob
This sucker has an innate Acc penalty and a hefty End cost, so be sure to slot both, then add some duration to it. The recharge time is fine as-is. In the early game this will keep mobs rooted for Freezing Rain, followed by damage AoEs to take down large groups. Later on you can combine it with Thunder Clap to Stun/Root mobs when GDF isnt up. Whenever possible, try to make sure someone else has aggro before you cast this, or youre liable to get wasted by a salvo of ranged attacks from the mobs you just rooted.
Slot[07] Level 10 : Hover /DefBuf
Opens up the Fly pool. I choose Fly over other travel powers because it provides perfect 3-dimensional mobility, which is a godsend to a Controller who want to not only stay out of melee combat, but also needs a clear view of the entire combat. This is vital when you have to make judgment calls mid-fight of where your help is needed. If you choose to go another route, thats all right too, but you may find your combat mobility hampered.
Slot[08] Level 12 : Swift /Run
Hurdle is good too, but with Fly as my travel power I prefer Swift, since I dont lack for vertical mobility. If you choose to go the Super Speed route, Hurdle is a better choice.
Slot[09] Level 14 : Fly /Fly,Fly,Fly,Fly,Fly
This is my own personal opinion, truly. If you dont like the idea of investing heavily in your travel power, feel free to place the slots elsewhere there are plenty of powers in this build that would benefit from extra oomph. Personally, I find Fly far too useful to give up and am willing to sacrifice a few slots to maximize its utility.
Slot[10] Level 16 : Freezing Rain /EndRdx,Slow,Slow,Rchg,Rchg,Rchg
This is another of your frequently-used powers, and there are a number of schools of thought on how to slot it. I choose not to slot Defense DeBuffs as most heroes have already ensured their own Accuracy is up to par, and I boost my team further with Tactics, making Defense a moot point. Two Slow enhances will nearly reach the minimum movement speed cap, which is useful since enemies usually try to scatter out of the Rain, and three Recharges plus Hasten will allow you to chain cast it with very little downtime, so you can keep the mobs debuffed constantly. The last slot is up for grabs I like an End Redux as the power isnt cheap to activate and I chain cast it to magnify my teams damage output.
Slot[11] Level 18 : Gravity Distortion Field /Acc,Hold,Hold,Hold,Rchg,Rchg
This is the defining power in the Gravity set: ranged, targeted AoE Hold. It has a 2 minute downtime, so slot it with several Recharges to cut down on that; permaHasten helps a lot, too. A lot of people slot it with 2 Acc enhs to make up for the inherent to-hit penalty, but I find that using one, coupled with Tactics, is as much as I need, which allows me to use three Holds in it. This will keep a pack of +3 mobs out of commission long enough to wipe them all out with little to no risk to the group, which is the point. If you find yourself moving from pack to pack more quickly, switch out a Hold for a Recharge, but I find this configuration ideal.
Slot[12] Level 20 : Health /Heal
Second step on the way to Stamina. The difference is minor, but noticeable, and cuts down noticeably on your downtime between fights.
Slot[13] Level 22 : Stamina /EndRec,EndRec,EndRec,EndRec,EndRec,EndRec
This is really a must-have for this build, since the powers I use most often are not cheap and I run several toggles all the time. Its not necessary to six-slot it until you can buy Recovery SOs at level 30. Once that happens, however, maxing this skill out should be a priority.
Slot[14] Level 24 : Maneuvers /EndRdx,DefBuf,DefBuf,DefBuf,DefBuf,DefBuf
The buy-in to Leadership. What we really want out of this Power Pool is Tactics, so you have to choose between this and Assault. As Assault cant really be enhanced at the moment, Maneuvers is the better choice. Coupled with Steamy Mist later on, this will help us protect ourselves and our team from enemy attacks, especially the vulnerable Blasters and Defenders that you will most likely be sharing the backfield with.
Slot[15] Level 26 : Tactics /EndRdx,HitBuf,HitBuf,HitBuf,HitBuf,HitBuf
This ones a winner. Slotted this way, Tactics can replace a single Accuracy SO in all of your powers, freeing those slots up for other enhancements. This effectively gives you an additional slot in every targeted power you have. Its also a boon to your team, since they all get to benefit from the boost as well. I always run one End Redux in every toggle power I have, to cut down on the End drain.
Slot[16] Level 28 : Hasten /Rchg,Rchg,Rchg,Rchg,Rchg,Rchg
This build doesnt benefit from Hasten as much as many others do, especially damage ATs. You wont really notice the difference until youve got it running permanently. Once that happens, however, it serves much the same purpose as Tactics does, by effectively adding a Recharge SO to every power in your arsenal. This makes a huge difference for powers like GDF that have long recharge times. The defense bonus is negligible but a nice bonus.
Slot[17] Level 30 : Hurricane /HitDbf,HitDbf,HitDbf
A favorite of all Storm Defenders, but we get it too. The repel effect is awesome for keeping melee types off you. Coupled with your variety of Defense boosts, this power will let you tank groups of mobs and tear a bosss Accuracy to shreds. Keep in mind, however, that you can still get hit, and unlike a Tanker you dont have many HPs to spare. Be careful.
Slot[18] Level 32 : Singularity /Dmg,Dmg,Rchg,Rchg,Rchg,Rchg
One of the best pets in the game. With Hasten going, four Recharge SOs will pretty much put you at the recast cap, so go ahead and slot a couple Damage enhancements in there. This will still allow you to keep three Sings up at all times, which makes for a phenomenal amount of fire-and-forget control flying around and can make normally deadly fights seem easy. Singularities are incredibly accurate, so Acc enhancements are unnecessary. If youre not interested in damage, then consider End Redux, as the cost is hefty.
Slot[19] Level 35 : Thunder Clap /Acc,Dsrt,Dsrt,Dsrt
This helps make up for the fact that Gravity doesnt have a second AoE incapacitate. The recharge time is 45 seconds with Hasten thats closer to 30, so Recharges arent necessary. Coupled with Crushing Field you can Stun/Root a group of mobs when GDF isnt up, which is a godsend in drawn-out fights or when moving quickly from group to group.
Slot[20] Level 38 : Steamy Mist /EndRdx,DefBuf,DefBuf,DefBuf,DefBuf,DefBuf
This is basically Maneuvers with a minor Stealth component and Resistance against some elemental and energy damage. Quite nice, really. The resistance isnt worth slotting, but max it out with Defense Buffs to boost your group. As always, End Redux for a toggle is a good idea, and especially so for this power, as it costs more than other toggles.
Slot[21] Level 41 : Super Speed /Run
A nice travel power, and the other half of our Stealth combination. Coupled with Steamy Mist, you become effectively invisible to anything short of a sniper. Use this to jump into the middle of a group of mobs without being noticed, then drop Thunder Clap/Crushing Field as an alternative to GDF. Also good for quickly moving around with your toggles up, when Fly might be too much of a drain on your End bar.
Slot[22] Level 44 : Lightning Storm /EndRdx,Dmg,Dmg,Dmg,Dmg,Dmg
Mostly for show, but its not as bad as most people make it out to be. It hits fairly hard, and the knockdown is useful for when things go bad in a fight and you need every tool you can get. Coupled with Singularities and you can even solo fairly well, though large groups are a risk. If youre worried about it missing too often, switch in one Accuracy, since it doesnt benefit from Tactics.
Slot[23] Level 47 : Wormhole /Acc
At this point, there arent many decent powers left to pick up. Im not a fan of Wormhole, but Update 2 has seen its range increased significantly, which make it slightly more useful for separating individuals from a group at the start of a fight. This can also be used as a poor-mans control power by slotting it with Disorient enhances and using it as a single-target Stun. Really though, you have better ways of doing that. This power, and the next, can be replaced entirely by something else, if you prefer.
Slot[24] Level 49 : Vengeance /Heal
Generally considered a piece of junk by most. Ive fought against enough Archvillains to respect their ability to wipe out even high-level teams, however, and I consider it my responsibility as group controller to try and keep a bad situation from spinning completely out of control. The group heal/def/acc/dmg buff is a nice thing to have, and might prevent a single persons death from becoming a group wipe. As with the previous power, however, this can be replaced by something else entirely, if you prefer.