Others have already brought up some good points about gameplay, lag, etc, and even touched a bit on teamplay and coop.
Yes, of course the game has to be fun if I play it, but the single most important factor for me is the likelihood that I will be able to play with my friends if I play this game. If, when WoW comes out, a large percentage of people from AoH and from here are playing it, I will buy the game. If not, I won't. That puts me in a very passive position as far as game choice goes, but I need to be realistic about what I enjoy in a game. I like to be able to hop online, and, with a minimum of fuss, find people to play with who have a similar attitude about gaming. If I can do this, I'll buy the game, if not, I'll skip it.