You have heard Blizzard's motto for raiding in Wrath of the Lich King, "bring the player not the class." The intent is to steer away from strict raid composition and shake things up.
Encounters are being designed so that no one single class is necessary, although the 25-man Razuvious fight currently requires at least one priest, preferably two or three, with at least one specced shadow.
Classes are being designed so that many necessary buffs, such as Replenishment, can be acquired through a variety of classes, rather than just one.
The rigidity that came with Sunwell is one of the reasons that development has taken this direction. The motto sparked high hopes, and not all players are convinced that the implementation has been successful.
In response, Ghostcrawler points out that just like everything in WoW, it is a process. It's not going to be perfect, because things are always changing.