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1) Sort of, yes. Controllers now get Containment damage, basically doubling their damage against enemies that have been controlled previously (either with their powers or any others). This can help their damage output significantly, depending on the set. Defenders just got a bit of a boost when their solo/on a small team. They get a 30% damage boost when solo, going down 10% for each team member they have, down to 0% for a 4-man team.
2) Low-level population is fine, since people are encouraged to make alts and try out new characters. However, a large portion of new characters are being made in Praetoria, which requires you to have GR to access. Still, you shouldn't have a problem finding teams. Also, super-sidekicking has been introduced, meaning that you can basically team with anyone of any level, and gain XP while doing so.
3) Pretty much, yeah. However, you may want to check out Inventions, which can sometimes be useful before level 20.
4) Check out ParagonWiki. There is a page there that will go through all of the updates you may have missed, and is a great source of info in general.
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http://paragonwiki.com/wiki/Issues
Note, wiki.cohtitan.com is also the same site (avoid wikia though).
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Welcome home - you might notice one or two changes to the game since you were last here 
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I also highly recommend downloading Mid's Hero Designer. You can play with the builds, look at the numbers, and get a real feel for the toons.
Also, if you liked Defenders but felt the damage was too low, you can check out Corruptors since you have CoV. You'd probably want to start it in Praetoria once you get the expansion, but you can try it redside to get the feel for it. Scourge definately helps the damage once you get a foe down to low HP. Although ultimately the damage is not THAT much greater. (If you have lots of damage buffs/res debuffs, the Defender can end up doing the same damage)
The main advantage of a Corruptor is that it's a lot easier to build around its offense, concentrating on the attacks, and picking up the offense boosts when they open up, without feeling guilty about putting off the more team-oriented powers.
1) A bit, the defender's inherent (Vigilence) was recently modified to include a damage buff based on team size, in addition to the endurance discount for hurt teammates. And controllers have Containment, which allows them to do extra damage to mezzed foes.
But both are still low on damage output compared to other ATs, considering they fill a support role more than offense. That said, some powersets (usually anything fire) do tend to do extra damage as their secondary effect, and Kinetics is VERY good at boosting your (and the rest of your team's) damage output.
2) Not that bad, but a lot of people are rolling their new characters in Preatoria (going rogue). If population is a concern you might want to consider the Freedom and Virtue servers. The first has a bit of a reputation for attracting twits, and the second is the unofficial in-character roleplay server (roleplaying is not necessary, but you have been warned).
Any of the other servers are still perfectly fine too, and just remember, you don't have to necessarily team up with like-leveled characters. Super-Sidekicking ensures that all members of a team fight at the same effective level (+/- 1). Only thing you have to keep in mind is that a lvl 10 SKed up to lvl 40 might not have the tools to effectively deal with the threats lvl 40s regularly face, but the lvl 10 can still hold his own with some help.
3) This is still somewhat true... although individual tastes vary. Important levels to remember are lvl 12, when you can start slotting lvl 15 Dual Origins; and lvl 22, when you can start slotting lvl 25 Single origin enhancements. At lvl 30, common Invention Origins are about on par with even level SO, and lvl 35 IOs start surpassing SO effectiveness. IOs do not degrade as you level up.
Generally speaking, I think common wisdom for enhancing as you level involves slotting Training Origins as they drop (if at all), and selling the rest. Then at lvl 12 use dual origins to enhance accuracy (as beginners luck starts to fade), and the rest can be left to slotting what drops you get. Then around lvl 22, either start slotting common IOs or SOs; the SOs you need to replace every 5 levels or so, while the IOs you won't NEED to, but you'd want to start replacing them with higher level ones starting at lvl 32 as funds allow.
And you may want to read up on Enhancement Diversification. Essentially, slotting more than 3 of the same SO in a power is in most cases pointless. Common slotting for most attack powers is 2-3damage, 1-2accuracy, 1-2recharge, 0-1endurancereduction. Endurance and Recharge may switch importance depending on how often you use the power and how much endurance it costs.
4) you've probably noticed all of my links point to Paragon Wiki (http://wiki.cohtitan.com/ or http://www.paragonwiki.com/). It is the best resource on the game around. bookmark it. Specifically pertaining to your question here; an overview of the 18 issues of free content updates.
Other resources of note are:
- Badge-Hunter/Vidiotmaps for all your map and badge-tracking needs.
- HeroStats - 3rd party utility for detailed tracking and analysis of in-game stats
- Mid's Hero Designer is a popular build planner
- the Titan Network hosts many of these resources, and several others I haven't mentioned
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As far as #3 goes, it is quite easy to get plenty of Influence/Infamy just by doing a few simple things in the market. Then there is no reason to deprive yourself of any enhancements you may want.
All you need to do is find recipes or salvage that have a bunch for sale and no bidders. Bid low (1, 10 or 100), fill up and then run to a vendor to sell. There are more sophisticated ways to make influence, but this is easy for anyone to understand. King's Row is a great place, since the market is right next to a vendor.
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I haven't played for a couple of years, and even then it was only for a couple of months after a previous long break. I have several low-level heroes (highest is level 17 /blush), so I figured it would probably be best to just start over. I have the CoV expansion, but I don't have GR yet.
1. I liked my defender and controller quite a bit, but I did suffer from low damage output. Has that improved any?
2. If I roll another defender, soloing may get tough. What's the low-level population look like for grouping these days?
3. As I recall, conventional wisdom in the past was to avoid spending influence on slotting powers until level 20 or so. Is that still a good idea?
4. Are there any good FAQ's available to catch someone up (heck, more like educate all over again) on all the changes since launch?
Any other tips or advice would be much appreciated.