I dunno, I guess it's alot of different things for me, but outside of personality, the things I'm probably most likely to notice first in a girl would be her eyes, and hair.
I've kinda had this thing where the last sevaral girls I've asked out all fell into the same category, as being alot of fun, down to earth, and into alot of the same things I've been into. Oh, and crazy colored hair, that was always a turn-on for me xD
Unfortunately, Most of the girls that fit into that category have turned out to be lesbians...
Physical attration is important to some extent, though; I mean, she has to at least be bareable to look at xD
Then again, I'm more likely not to make such quick judgement, and try to get to know a person before I just blindly ask them out.