Aside from the obvious physical attraction... big lips, breasts, butt, legs, etc?
Basically what the other two guys said here thus far... it all falls down to a girl making a guy comfortable being around you, giving you the "right" to be himself. In other words, a good sense of humor is extremely enticing in a girl's personality. A playful spirit, which isn't detered by vulgar jokes, is an addition. Ask any soldier, for example, what girls are you most comfortable around, they'll likely tell you the "girls that blend in with us boys".
The fact that she can make me smile as much as I do to her. If I'm attracted to her I will feel for her pretty good. If she is overall just a chill girl and I can talk to her about anything is when I will start to fall