Rezzing this thread I made awhile back, because I think the information could be useful to newer players.
Cool guide and diagrams 
I agree, it could be very useful to help someone understand LOS and how it relates to corners and elevations.
Good info on corners but I've got to echo Dork Man, why take the high ground? The few heroes that do run do not run slower uphill or faster downhill. Your ranged attacks don't shoot further. Really, for purposes of this game, the high ground just doesn't matter. Now a cliff, on the other hand, is another kind of corner and should be treated as such. Hilltops can be, kinda, if they are somewhat sharp. If you have two supers facing off against each other on the same side of a large hill, though, it isn't likely to matter much. Even on the very unlikely chance they stay on the ground, about the only advantage I can see the higher player having is that knockback will be more likely to give a little falling damage.
By taking the high ground, you have a line-of-sight advantage. Essentially, any cover which is LOWER than your position looses effectiveness since you can look (and shoot) over top of it.
PPD X-45, 50 Claw/Will Scrapper
"We're sworn to protect The City. And we're just going to have to face it: that includes the sewers."
- The Tick
Interestingly though.... and I've only seen it happen in PvE... but I figure it should work for PvP since its simple game physics...
if a unit is on the high ground of a steep hill. (Such as a sewer downpipe, or steep bridge)
and is hit by a knockback effect, then that unit slides down the slope for quite a while before retaining its footing.
The high ground is good... unless you are sniped off it.
But you are right the importance of terrain, even in a mmo like CoX can't be stressed enough
"If you know Heaven and know Earth, you may make your victory complete." - Sun Tzu, 'The Art of War'
As a blaster, I have found it necessary to develop a set of tactics, most of which I derive from the RTS game, Rome: Total War. http://www.totalwar.com/
It dawned on me one day, while fighting in the PvP arena, there are some people who don't know how one's position can effect battle.
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Epoch: Yo, let's take the high ground!
Dork Man (not actual name): Why?
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In this post, I shall explain some useful tactics for utilizing the terrain of the game.
Rocks/pillars/trees/ect...
When the dev's created the world of Paragon City, they scattered rocks and other obsticals all across the world they had just created. What does this mean to you? Well, it means that 2 characters (in the rest of my little guide I shall call them units) can't begin an attack on each other until one of the units goes around the object and into the range of the opposing unit. This fact is useful for herding, strafing, ambushing, and keeping out of the reach of monsters with big teeth.
In pvp, if one's opponent is targetting using the 'mouse-click' method, hiding behind rocks can help make targetting one's unit more difficult, like a palisade. If one is a ranged fighter, one may be able to get multiple shots at one's opponent over the tops of rocks before one's opponent is able to retaliate.
Corners
Corners can be very dangerous for a hero, because one doesn't know what's behind the corner until one gets close to it. However, corners can be useful for herding, strafing, and ambushing. The important thing to know about corners is what I shall call, line of sight.
Line of Site (Epoch's definition)- Area in which an unit can visually perceive an object.
Corners impede view, that can be both useful and hinderful.
In the following diagram, the enemy line of sight is shown in red.
(note: in this section all diagrams are shown from an aerial view)
http://img.photobucket.com/albums/v6...rcass/los1.jpg
Basically, when dealing with corners, one's chances of being spotted are reduced the closer they are to the wall on the side the enemy is on.
In the next diagram, notice how the player's line of site (shown in blue) is not obstructed when they are close to the edge.
http://img.photobucket.com/albums/v6...rcass/los2.jpg
My point of this section is, it's wise to hug the wall when moving through corridors.
Choke Points
Choke points are enclosed spaces in which an enemy unit must pass through in order to enter a range in which the enemy can effect one's unit. Choke points can be utilized in herding, strafing, ambushing, and dying!
The problem many teams have with choke points is that they can become clogged, thus preventing passage through them which can lead to deaths. But choke points can be used as valuable and dangerous tools, one and one's team just needs to know how.
The following diagram depicts the enemy's line of site (red) when they're on the side opposite that of one's unit. Notice how the enemy's line of site is severely limited.
http://img.photobucket.com/albums/v6...rcass/los3.jpg
That means that there is only a certain range in which they can attack one's unit.
Many teams handle choke points by clogging up the choke point with a damage taker such as a tanker or scrapper, but that prevents the damage dealers and debuffers from utilizing their full potential as they suffer from the same point of line handicap as their enemy.
Another method teams use is to let the enemies through the choke point, but then hit them directly from the side as they enter. For this to work, one's unit must be against the barrier. This works because one has an oppertunity to hit an enemy before the enemy's line of site fills the room and one's unit becomes a target.
http://img.photobucket.com/albums/v6...rcass/los4.jpg
The High Ground
In most historical wars, the army with the high ground usually has a tactical advantage over the lower army (the exception to this rule is WWI, but even then Zepplins, make-shift bombers, and recon planes were advantageous). It was this logic that would lead to the building of castles and fortresses (which General Patten once referred to as the testiment to the stupidity of man). The problem with such fixed positions was that they were not mobile. But luckily for you super heroes, unless you're held, you have mobility and it is that mobility which allows you to utilize the high ground.
Note: All diagrams in this section are from the side.
It's simple, he who holds the high ground controls the tempo of the battle.
If one stands by the edge of the vertical plane, one can shoot and recon to one's content, but at the risk of being shot and reconned back.
http://img.photobucket.com/albums/v6...rcass/los5.jpg
If one stands away from the edge of a vertical plane, then one is protected from being shot at or seen (allowing for hidden manuevers), but at the cost of being able to attack the enemy.
http://img.photobucket.com/albums/v6...rcass/los6.jpg
As you can see in the diagrams, the closer the lower unit is to the rise, then the less likely it is that unit will be able to be targetted by the higher unit.
If the enemy intends to charge up the incline, then the enemy exposes itself to attack.
William Wallace, ancient scottish general made famous by the movie Braveheart, destroyed the British army by rolling boulders down hill on them (although that's not what they did in the movie).
K, hope ya'll got something out of my little guide.