First things first, the physical. A girl with a shapely body and a beautiful face is an automatic turn-on. Bonus points if she knows how to show off her body in a tasteful manner. What constitutes tasteful, however, can vary from guy to guy.
The right attitude is hot. One that shows a girl is comfortable with herself and her sexuality, without being brazen. E.g. - guys don't want prudes, but they don't want sluts either.
Attitude also covers her not being, essentially, a bitch. No guy wants to date a bitchy girl for long, although what exactly constitutes bitchiness can vary somewhat from guy to guy (for instance, a sarcastic sense of humor - some guys like it, some don't). Generally though, it's agreed that stuck up, backstabbing, or just downright mean girls are bitchy. And just so you know, even the hottest girls can become fairly unattractive to guys if they're bitchy.
Pheromones - these do play a role. I've met girls who, while otherwise nothing special, somehow managed to be extremely sexy and attractive to guys. This is more of a you have it or you don't kind of thing though.