The fact that people are actually AVOIDING playing the WotLK beta should tell you all you need to know about how much content they think the next expansion will provide.
For me personally WotLK does less than nothing. Another few tiers of the same old raiding game I finished in Azeroth, more grinding of a broken PvP system, and just ten levels of what actually made WoW great, questing and exploring new zones. If WotLK existed in a void, I would be looking forward to it just to see the few new zones, but sadly it does not. EVE is currently keeping me more than busy, and that little game called Warhammer Online is just around the corner. Between those two, and what each offers, the changes that WotLK will bring look very weak, and I think we are starting to see that feeling creep around the MMO space.
Will it do well? Of course, it’s still WoW, and clearly enough people have shown a love for the grind to do it all over again. But do I believe a significant amount of the player base is tired of TBC+1 in WotLK? Yes. And it’s that player base that helped AoC get the launch numbers it got (before they all left after the first month), and it’s that base that will at least give WAR a shot. Whether they drift back to WoW or stay in WAR just comes down to Mythic delivering on their promises.