quility's got an interesting dicussion/unscientific poll on these questions: what causes you to attend a skiffy convention? what caused you to go to your first Con? And, finally, if you have ever helped run a Con - what caused you to take that next step?
I posted my answers at the bottom there, though I guess I didn't say what draws me to a con now, which involves what friends might be there, proximity to home or other friends and relatives we'd like to see, timing, cost, and also GoHs and con reputation (I have a small list of cons I'd like to check out because I hear they're consistently good cons, and I consider Worldcons in part based on their committees).
(ETA the specific link)