BIG TITS is hardly idiosyncratic or quirky, but I love 'em. More to the point, I love NICE breasts, and if they're both big and shapely I'm certainly not going to complain. Before I read the OP, I actually thought this was going to be more about sexy personality traits.
Really short, spiky hair is sexy on the right woman (think suicide girls). A subversive personality and an "I love nerds" T won't hurt her, either. Though it's a trite sitcom, I still sometimes enjoy watching That 70's Show, and my favorite part was always when Donna said to Eric, "Oh, you're my horny nerd-boy!"
Funky taste in music, a bit of an artsy disposition without being pretentious, and a TRULY liberal outlook (i.e. not just identifying herself as a feminist b/c she's angry at men) are all pretty damned close to being must-haves. A colorful wardrobe is also a turn-on, especially if it's of the sort that says "I can enjoy being a sexy ***** without being a ho."
Being truly bi is also a turn-on, and no, it's not just for the obvious reasons (anyone who's ever inhabited a liberal arts town for any length of time knows what I mean). The sort of bi woman who's actually interesting to talk to because of the specific things she likes in girls - and they're also better lovers, btw.