I play City of Heroes/Villains on and off. I've tried a ton of MMOs, but none have lasted more than a month, except for CoH/V that I've racked up 16 months since it was released. The reasons:
1. Amazing story. You don't go out and kill 10 wolves because there's too many of them, or deliver soup to some dude who's standing watch somewhere. You stop a nefarious supervillain from building a megarobot he will use to take over the world. Missions aren't pointless and annoying.
2. Fun combat. You don't pull a character then gank him to death with your eight man party. You leap into a group of dozens of baddies and beat them up. You actually feel heroic.
3. Unique characters. In most MMOs everyone looks pretty much the same. Not so in CoH/V, in this game you can spot someone you know a mile away. No two characters look remotely the same, unless you've got a really poor imagination (oh look, another Captain America clone). Even then you'll easily tell them apart when looking close.
4. Every class is fun. In most MMOs I've found maybe one or two classes I enjoy playing. In CoH/V I have 35 or so alts. Then again, it does have I think around 2-300 different combinations to try...