I think the idea of a WoW killer is a myth. I've used this analogy a few times before, but it's still completely true: WoW is the king of MMOs in the same way that Budweiser is the king of beers. It hit at the right time and the right place and made itself a cultural icon, and nothing is going to take it out. It's definitely not for refined palates, but nothing will ever be able to reach its heights in popularity no matter how good they are, because it is the king.
Instead of trying to topple WoW, other MMOs should strive to be like the craft brew market, putting out a superior product and going after the smaller, pickier crowd. There are beers I'll pay twenty bucks a case for. (Ever hear of Wild Heaven? or Terrapin? or Stone? *drool*) Likewise, there are games I'll still pay $15 a month for.