Not much is said about what probably is one of the biggest factors in whether something turns out into a good or bad performance in WoW - movement. Be it dpser, tanker or healer, the way you move in a raid setting is often crucial for the outcome of the fight. When I finally down a tough raid boss my first thought often is that we finally got all the moving right.
Unholy Dk's once had a talent that made Unholy Presence an Aura. Everyone in the dk's raid gained some runspeed. Blizzard had to remove this talent (or actually redesign it) because it turned out that runspeed was just a too powerful stat. Depending on how mobile a fight is, some extra runspeed could increase the dps of the raid by several percent. Movement is a power not to be trifled with, it can be used both in very good and very bad ways.
Movement is tricky, because it is so highly dependable on what situation you're in. But as with everything else there are pointers that need to be in the back of your head all the time if you wish to maximize your performance. Most people in a serious raiding guild have a good idea about how their classes work. I therefore often find it interesting to see how otherwise skilled players can fail miserably when it comes to moving around in a proper way.
When I do a particularly difficult encounter, like Putricide HC or Sindragosa HC, the raid rarely fails from lack of healing, lack of dps or bad tanks. The raid fails because people fail to move. They don't handle the disease properly, they don't move from the Frost Nova, they don't stand behind Frost tombs properly etc etc. POsitioning and moving in a good way is essential for you to survive and by that for the raid to succeed.
What you have to think about when moving depends on what fight you're fighting, as mentioned, and what class you're playing. Some classes have different responsibilities and also move in different areas of the fight in which they have to keep an eye out for different things.
Tanks often have to move with the boss in consideration. Their movement therefor often revolves around making the boss as available to take damage, and unavailable to deal damage, as possible. The tank often has the unique position of standing in front of the boss, which also often includes making sure that the boss is aimed away from the rest of the raid and positioned so that whoever wants to hit it doesn't have to stand in bad stuff. Tanks are often treated in a special way by game mechanics and therefore don't have to think about avoiding or handling the same problems that the rest of raid does. This can either be because the (main) tank isn't affected by a particular spell, like Necrotic Plague on LK, or because they don't have the responsibility to handle it, like the spores on Festergut. This is because the above mentioned positioning of the boss often has prio over these kinds of mechanics. If the tank had to run with the plague, the entire raid would suffer since the boss would be moved to an unwanted position.
Sometimes the main tank and the off tank have completely different duties, like on Rotface or Putricide phase 1 and 2. On most fights however they do the same thing but at different times or places (like taunting off eachother or tanking adds).
It is important to note however that tanks are the only ones in the raid that should have this kind of behavior towards the boss. Unfortunately one of the first and biggest mistakes I see dps do is prioritize movement towards the boss over movement towards other mechanics of the fight, like if they were tanking. A disease/nova/other shit turns up and they somehow think, or don't think, that whatever they are doing with the boss is more important. Nothing could be more wrong of course. In fact, dpsing the boss is usually the least important thing to do right in a fight. Eventhough most bosses in ICC have an enrage timer, it seems to be there merely as a formality.
A good example is yet again the Putricide fight. Say a melee gets the disease, he is then supposed to run towards a ranged to get rid of it. Optimally he would keep the debuff for as long as possible before trading it, seeing as he got the debuff-debuff anyway. Few dpsers do this. Getting back to the boss to continue dpsing is apparently way more important, eventhough one often halts dps in this fight to time phase transitions. Another example is when the oozes spawn. It often takes many, many wipes before ranged dps get the idea of stopping to dps on the boss -before- the ooze spawns so that they can position themselves optimally and start dpsing the ooze as soon as it appears. Does not doing damage really hurt that much? But maybe I'm moving away from the subject somewhat here (pun intended). Point is, tanks are moving with the boss position in prio, but most of the time no one else should.
Most dpsers have some kind of cooldowns to help them move around in a faster way, like blink, sprint and disengage. Many of these even show how important movement is since they were implemented to make some classes less immobile, like Feral Charge - Cat, Charge and Shadowstep. There is a big difference between ranged, caster and melee dps however. Caster dps have to, just like healers, always consider their skills when they're moving around since most of them have cast time. Melee have to put more focus into being in range than a ranged class has to, something they often point out and which one also often notices in fights where one has to move around alot and switch targets. This means caster dps might want to use instants when moving, even if it means clipping dots. Melee have to check that they're standing within range and preferably behind their target throughout the fight, something ranged rarely do. Because ranged don't have to consider these two factors as much as melee, they can switch more readily between targets, something I personally think they use less good than they should and could.
Melee and ranged also have two totally different areas to move around in. Melee often has this small position huddled up in the boss' ass or side, while casters can move around more freely in the room/area. This means melee have a special relationship with the tank since their dps is depending on how the tank is positioning the boss (see above) which yet again, the ranged don't have to think about.
Healers have a unique movement pattern as well (everyone is unique!), since they're not positioning themselves for the boss at all, but for the rest of the raid. I often think mobile healers like druids and priests often use this freedom way less than they should. Running around does include some extra responsibility and awareness and in most fights you're not even supposed to move much at all, but I often think that keeping a mobile attitude towards the fights where running around is needed makes things easier. On LK phase 2 and 3 or Putricide, I often find it easier to handle different situations if I run around all the time instead of standing still. If I stand still I might react too slow when moving out of a Defile or moving away from a Malleable Goo. If I am constantly moving I am automatically moving away from the danger while it is coming towards me (this of course means that you might run into danger instead, but since the dangerous stuff in Wow often is really visible I find this to be less of a problem). By keeping a mobile mind I can react faster to new situations. For larger areas this also means keeping in touch with the entire raid, if they're spread out. Sure you'll get out of range with some, but in range with others and then back again. For someone with alot of instants like me it is the best way of healing.
The more immobile healers have to think about other things. Often this means positioning themselves so that they can do the big heals undisturbed when they're needed. Paladins and Shamans therefor often prefer to place themselves smack in the middle and tell everyone to stay the heck away. On fights like Festergut these classes also are favored in that the more mobile classes come to them instead of them having to run around, just as with the tanks. Holy Paladins are often glued to the tanks and move whereever they go (I sure hope holydins have a brighter future in Cata btw), which means they always have to think about where the tank is.
*phew* There sure is alot to be said on this subject, and I have really only touched the surface. The point of this post was to highlight something I feel few people seem to take into enough consideration today. Maybe I am unlucky, but like I said at the beginning I often get the feeling that the fact that a difficult raid encounter was overcome is because people were lucky in their moving around. It's like throwing a bunch of marbles into a board with holes and if you're really lucky most of them won't fall down. So if you try often enough you will succeed. What if you could decide how to move the marbles (omg would that be like pinball!)?
Dpsers often prioritize dps over moving, and rarely realize that they're really the same thing. If you don't move in the right way you'll die, and if you're dead you sure won't do much damage. Tanks often move as they should since everyone notices at once if they fail with this. But their movement comes with the biggest responsibility since their failing results in a wipe rather in just them dying. Mobile healers don't use the fact that they can position themselves more freely as much as the could. Immobile healers are just that, immobile. Sucks to be them. But it also means having to keep track of when the right time is to move in a whole nother way, just as caster dps really.
So them WASD (I actually prefer QWES) buttons are more important than you probably think.