I don't play WoW, and am generally not a fan of MMOs. I've tried trial periods on several of them, and never end up subscribing. This is in contrast to single player computer and console RPGs, which I love and buy tons of. The difference boils down to two reasons:
1) I have a firm principle against pay-to-play subscriptions. My gaming time is too inconsistent. I'll have months where I play all the time, and months where I hardly play at all. Unless I really, *really* like the game, I'm probably not willing to pay-to-play it. The only MMOs I've played all the way through were the Guild Wars series, precisely because they weren't pay-to-play.
2) It came up in other thread here recently: I am an *extremely* story-motivated gamer. I play single player games on the easiest difficulty (or with cheats turned on) because I'm not interested in the challenge. I play because I want to find out what happens next.
In my experience, MMOs are particularly bad for my playstyle. They generally have shallow, under-developed plot lines in contrast to single player RPGs. Instead, the big motivator is supposed to be an urge to prove your awesomeness by collecting ultra-rares/defeating uber-bosses/whatever. And that just doesn't interest me at all.
I'm also not really interested in the "interaction" that MMOs foster. I'm not interested in playing with dedicated guild members, because I'm simply not dedicated enough. I don't want to deal with their expectations of me. And, IME, playing with pickup groups is worse than just playing a solo game, since I'm dependent on the the behavior of random people I don't know for my enjoyment of the game.
These two combine to mean that, after reaching the end of the plot line in an MMO, there's simply nothing for me to do. The whole "endgame" experience is incredibly pointless to me.
That's what I don't play WoW, why I stopped playing GW once I beat the storylines, and why I don't play any other MMOs either.