I played "World of Warcraft" for a time, but I got out because I was getting sick of it. The grinding was becoming tedious with little reward, the high-end content was out of reach because I didn't know a group of 39 people to raid with, and I just plain don't like pvp. That, and I didn't want to shell out $40 for an expansion just so my demon-slaying warrior could get his butt kicked by a snow pig.
I much preferred "City of Heroes" because the story was rich, it was very solo-player friendly, there was more customization and less grinding. I stuck with it for over two years (it helped that the "City of Villains" expansion was only $30 and had a free month), but quit a year ago because there just wasn't any new content coming out.
I tried "Champions Online" and had fun with it for a while but found it less solo-friendly, the story/setting didn't take itself seriously at all, and it just ran out of content in the first month (which is really bad for an MMO).