Adding new classes and races is for casual players. Raiders want more high level content.
So now Alliance have Shamans. Big deal. The high-end raiding guilds will be hitting level 70 content and still waiting for the people that rolled shaman to catch up to them, they'll probably be at lvl 60 the same time that others get to lvl 70. Are those lvl 70 really going to wait for those lvl 60s to get to lvl 70? Also don't forget that those lvl 60 need to get geared up to the same point as the lvl 70s.
Adding new regions and levels is cool, it trivializes the older content but that's okay. But adding new classes/races and at the same time adding new higher end content? It's only for two different demographics. Casual players like the new classes/races but raiders hate it because having to wait for a new class (especially if the company makes it essential for newer raiding) just gets in the way of the kind of playing they want.
Expansion #2 will be some new high-end dungeons (no level cap raise). New race and classes (this is the only logical time to add new classes, as they level cap isn't that high and the new dungeons gives people time to wait as people level up to 70 with the new classes. Also in this expansion NEEDS to be all new lvl 1-60 content (dungeons and lands).
Expansion #3 will be new lvl 60-70 content (dungeons and lands) and level cap raise with new lvl 70-80 content.