Hair, face, piercings, nails, legs, feets, tits...
If she has awesome hair but then turns around and looks like she IS the ugly tree... I go no further.
Good face, check for piercings in face and ear. I hate earrings and piercings and they are a quick turn-off.
Hands... freaky manicures with two-tone polish or fucking gay ass designs or diamonds or pretty much anything... I stop. Weird colors... well, no, actually jut nail polish is almost enough to turn me off a lot of the time. Another simple thing that can turn me off in a heartbeat.
I move down to legs... assuming she's not wearing a half shirt that exposes a muffin-top. Ass is usually too hard to tell with clothing unless it's skin tight and unless the ass is clearly business class (too big to sit in coach... reference?)
Good legs... check the feet, if they can be seen. Gnarly toes, polished nails, rings, whatever... I'm not a fetish person but for some reason my eyes are always drawn down there. Cankles... see ya.
Tits... obviously flat chested can only be over looked if everything else is just perfect. Flat chest is too much like an undeveloped child... or a boy. Creeps me out. Gotta have a C-cup minimum.