Depends on the girl as even subtle things can do it for me. Sometimes I'm not overly turned on by the girl being naked, other times some aspect such as a curvy stomach may really do it for me.
Could be one or any combination of:
Hair - rather like girls with ponytails running in lycra
Fragrance
Leggings, boots, short skirt with tights - generally feminine clothing. Even wrapped up in scarves, thick coat etc. can do it for me if she looks feminine.
Smile
Voice
Eyes
Shape of her lips
Curves
Personality - some girls make me feel relaxed especially when their persona rubs off on you i.e they're fun, non-judgemental, nice to everyone all the time, generally happy
Smaller breasts with hard nipples
Wetness, generally being turned on and enjoying herself, her orgasm, knowing what I'm doing is doing it for her.