Me, personally, it comes down to three factors: 

1) I hate sample posts. (HaveIevermentionedthisbefore?) 

2) Are the great characters taken, especially women? (There's just not a lot of great female characters in comics--subjective, I know.)

3) Can I jump into a scene with a friend from the get-go? It can be insanely difficult for me to keep focus and post a number of times before I can start collaborating; the very reason I do Group RPs to begin with. 

During the heat of the semester, like now, I can sometimes only average one good post a week. So if that's not an issue for anyone, I could join a new comic game.