The only two things Aion was boasting above WoW was possibility of flight and aerial combat. Granted WoW probably won't implement open world aerial combat any time soon (the possibility is there with aerial vehicles, but I haven't heard of them being usable outside of instances), WoW will close the gap of flight entirely with introduction of Old World flying in Cataclysm. As fun as Aion will be, it will not be offering anything new. So I doubt it will pull anyone away, seeing as most people don't play WoW because of lore, but out of social obligation and character attachment. (Yes, I said social obligation, and I mean it.) My prove is that most people in WoW don't even have a clue what the most basic lore is--hence they don't give a crap about what they are playing as long as they are playing something.
Personally, I wouldn't mind if WoW will shed some of these guys. The game will still be very much profitable with half the amount of current players, and chances are the half that is left will be a much better crowd.