Guys: I really, really like skinny guys. They can be a bit muscular and wiry as well, or not, no biggie and I'll still like them either way. But the skinniness is essential. Also, less importantly, long hair and glasses. Neither or these are make-or-break, but a guy who I already like will be made more attractive if he has either or both features.
Girls: Shortness. Don't know why but I only seem to like girls shorter than me. Unlike with guys, skinny girls aren't attractive to me; I like curvy hips, although have no preference for breast size. I also have a thing for glasses, pixie cuts, red hair or freckles on a girl.
General: Collarbones. Shoulders. Backs. Hips. Yum.