The laugh is always a big contributor. It allows for there to be more of a connection and if she's having more fun with you then you will subsequently have more fun with her. Also, if you get a girl to laugh, especially one you're interested in then it just gives you a sense of elation.
Her eyes and the way she dresses up in a pair on nice high heels that s what drives me crazy
I think it depends on the girl, but in my case there's always three things: Smile, laugh and eyes, mainly because those are the three things that will stay the same in life. When I think about it, that's what really counts, because a woman's body is going to change, therefore boobs/butt aren't such a big deal..