There are two things in CoH which I've never seen in any other MMO.
1. (This is the MOST important thing to me in an MMO) I can earn all the best loot in the game by myself, I'm never forced to team up for it. I cannot stress how much I hate having to depend on others to progress my character. In fact, in CoH, one of my few laments that every TF isn't soloable by every class. Furthermore, if I do want to team, CoH handles loot correctly; there's no way to ninja drops, or point to fighting over them, because the game just gives them to people.
2. The smoothness of the animations is, IMO, unmatched in any other MMO I've seen. Wow and CO were liking playing in knee-deep mud, or at .75 speed. I will never get tired of hitting a foe and see him literally fly across the street, or down the stairs, or off the roof of a building. It never gets old for me.
There are other reasons for me, but these are my top two.