I guess my big issue with MMORPGs is the sameness of it all. I don't mean same content within the game, or even across games. I'm talking about sameness of experience among people. Sure you're in this big world accomplishing these quests, but so are hundreds of other people doing the exact same thing. Sure some of the details differ such as class, skills and equipment, but fundamentally you're all killing the same rats, finding the same keys, and exploring the same caves. That's not a bad thing, but it's also not worth the $15/month either. A lot of the benefits you get from an MMO are starting to be seen in other games that don't need a monthly subscription. Things like Guild Wars, Diablo, etc.
And the game is just so huge it becomes overwhelming. I've seen screens/videos of level 70 people doing raids and the amount of icons and crap they have going on is just ridiculous. When you're using automatic macros and half the screen is filled with icons and status messages, that's more like flying a plane than playing a game.
Of course this is coming from someone that prefers HUD-less games.