Since World of Warcraft was released, each expansion has shown a pretty decent change to the way raids work.
Originally, World of Warcraft had a 20 and 40 man setup.
Burning Crusade changed it from a 20 and 40 man setup, to a 10 and 25 setup, each having their own instances. Probably the most controversial change when it was announced
Wrath of the Lich King kept the 10 and 25 man design, but instead of separate instances, each one had just a different version. Hard modes were also introduced.
Cataclysm had probably the most minor change so far. Both 10 and 25 instances are supposed to be normalized, so they're the same difficulty, and award the same loot. So now instead of getting better gear from 25's, you can get the same from 10's.
So where do you think it'll go from here? I'm sure there will be some change. Personally, I don't see anywhere else for it to go. The raid setup has progressed pretty far, and I think Blizzard finally likes the place it's in. I know they mentioned a Random Raid Finder, so hopefully that's successful for alts and pugs.