Unfortunately I've never been able to stick with any MMO for any extended amount of time. I just lose interest. Probably the longest I ever got into an MMO was Dungeon Fighter Online, it wasn't really that great, but it least felt like I had a little more control over the combat than your average MMO combat system. I hear a lot about Guild Wars 2 having really good, fluid combat, and I made a character on a friend's account to try it, but I wasn't really impressed at base level.
I know you're not SUPPOSED to be impressed at base level, the real excitement comes with higher level play, but I don't have that sort of time to invest getting to the point where I'll actually enjoy combat. The social aspect is nice, but not enough to keep me in if I'm not enjoying myself on a personal level.
It's weird, my best friend from High School is VERY into WoW, it's one of the only games he plays, (whereas I play a lot of different games in a lot of different genres) and he really likes it. I tried it out with him once through the free trial, got to level 4 after doing 3 quests for about an hour, and thought to myself, "This is really boring, slow, and not fun at all, I'm uninstalling this." and that was my experience with WoW XD