I don't see what keeps people playing MMO's. Is it the immature players who ruin the game? Or maybe the total repetition of endless quests and instances is comforting on some level?
It's the addiction. I myself played quite the load of World of Warcraft, but I could never play for hours and hours on end. One of my friends is an addict, and in the summer he would wake up at 11am, play World of Warcraft until 12am, and repeat. It takes over lives, but it is sometimes amusing.
I myself am quite looking forward to Star Wars: The Old Republic. It seems like Bioware has quite the MMO in the works.