Raid composition is going to change in Wrath of the Lich King. A certain part of this is obvious and inevitable: they're adding a new class. But there is another major factor that's changing. Currently, buffs and synergy are a large part of the reason you might choose to bring one class over another. Shamans' awesome Bloodlust/Heroism, for instance, and their Windfury Totem, has made it often a good idea to bring many Shamans to a 25-man raid, and almost mandatory to have at least one. Similarly, the fact that Warlocks synergize quite well with each other (Warlocks cause other Warlocks to do more damage), due to effects like that of Improved Shadow Bolt, has contributed to Mages being much less favored for Sunwell Plateau.
How is this going to change in Wrath? Ghostcrawler made another rather long post yesterday on raid stacking, and this is the essence of what he said:
- We want to limit the power of stacking raid buffs, like we limited the power of stacking consumables earlier.
- "We want the challenge of the encounter to be the fight itself, not collecting all of the buffs and debuffs you need to succeed." They don't want to nerf buffs, but they want them to be "less of a burden."
- Therefore, for most buffs, there will be multiple classes that can provide that buff, and they won't stack. For instance, you can get your magic vulnerability debuff either from Warlocks or from Death Knights; those two abilities will not stack with each other.
The ultimate goal of this is to cause raid leaders to want to bring players they like, or good players, and not feel like they have to bring certain classes to get certain buffs and debuffs; and also to help class balance. Of course, they recognize that certain guilds are going to strictly min/max in any case, but the idea is that the benefits of a few classes shouldn't be so overwhelming that you feel like you have to bring five of them.
It's an interesting move. Assuming they execute it well, and preserve class uniqueness and utility (as they are promising to do), I think it will be very good for the game. And as this is still beta, and they are actively collecting and implementing feedback, if it doesn't work well right now they'll have time to fix it. The Wrath development team seems (thankfully) much more responsive to feedback than the Burning Crusade dev team was.