The guild I was in, and a few others, along with some solo players left 6 months ago to play Rift. A pretty sick MMO that launched almost exactly 6 months ago. When you look at other MMOs, WoW just doesn't seem to have it anymore. Their best expansion, The Burning Crusade, is long since done and over. The graphics are horrible, they constantly recycle content, and it just became stale.
Rift is cool, you have SWTOR coming out, Guild Wars 2 which looks fucking amazing! Check out some of the videos, they are bad ass:
https://youtube.com/user/arenanetofficial
I think WoW is dying, and the release of these upcoming beasts will kill it. There will always be a population, but it won't be significant much longer.