I still play (30 months now) on the US servers, Triumph and Guardian mostly.
what keeps you going back? I love super heroes, but didn't feel like the experience was compelling enough
I personally like the Diablo-esque hack and slash style of game play. While the missions you undertake usually aren't terribly deep (though more recently created story arcs are orders of magnitude better), I like to get my variety from playing different roles (offense, defense, offensive support, defensive support, army of one, etc). I also like to continually run my character against the red line to see just how far I can take them.
I like that playing CoX has no overhead. I can log on, and be on a full team within a few minutes. I have friends that are really into WoW. But before every raid, they have to spend something like 2 hours preparing for it: tedious tasks like crafting potions, etc. I have another friend who plays FFXI. It takes him an hour to get a on a team, and its next to impossible to get one if you don't have the expected cookie-cutter build and equipment.
I'm perfectly fine in trading a more shallow play experience for freedom. I don't have to spend hours on end crafting, and I don't have to worry about keeping up with the latest loot. If I have a two hour window to play in, then I actually get to
play for the whole two hours. I can tackle pretty much any mission with any number of players with any team composition. These are the things that keep me playing.
More recently, several game updates have added a lot to the game like the new crafting system, new Task Forces, new story arcs, etc. I also play the game with my wife, so I always have a partner. We tend to play complimentary characters, which gives us a lot of freedom to do whatever we want.