I've been playing since the CoH beta (heck, I upgraded to the DVD edition, and got the CoV special edition). What keeps me playing is that it's great for the casual gamer who doesn't want to spend six hours camping a dragon spawn just to have a chance of killing it and maybe dropping the 00ber sword of 1337ness that everybody must have just to remain competitive. I can pop in for a half hour, do a mission, and log out if I'm short on time. For those of us who have a few hours to spare, there are task forces which take you and a team of up to 8 people on a special set of missions with interesting stories (or trials, which are generally shorter but tougher and more rewarding). Also, combat is very quick paced with little downtime. PvP is completely optional (you can go an entire career without entering a PvP zone), but is pretty well done (except the arenas; those inhale audibly). There are also 10 different character architypes for different styles of play.
Every few months, the developers put out expansion packs (called "issues" for the comic theme) with new content such as missions, powersets, features, and the occasional nifty fun thingie. What's better is that while most MMORPGs will charge you for this content, these are free (with the exception of the City of Villains game, but it's essentially an entire new game, and the monthly fee does not go up when you combine it with City of Heroes).
If you're on the border of whether or not to get into MMORPGs, give either game a try (or, get both for the interconnectivity
). You won't regret it.
...
I realize I never actually answered the question. I have about 30 characters spread over three servers. I generally play defenders and controllers on the CoH side, and while I enjoy all the CoV architypes, my main (and favorite) is a Robotics/Poison Mastermind (I named my bots after FIRST teams; may all people who stand in my way fear the mighty Tigerbolt!)