What she's wearing. If her clothes expose her body a lot, I'm more likely to notice her. Even if she isn't pretty or good-looking at all, the fact that her clothes aren't concealing much of her body is going to make me look to check if I think she's pretty or not. :p
If I find their face attractive, and if they come across as friendly and intelligent (i.e., not walking with a group of friends and waving their arms around while swearing in the middle of some dramatic story they're telling... huge turn off, lol).