Going by NJQT's definitions, for me it's innocent stuff. For some reason, ponytails, glasses, button-up shirts and skirts of the business/office kind are dead sexy to me. All of them or some combination of them; either way. Oh, and when a girl has a nice, gorgeous set of hips and a booty to match, and she wears something that is perfectly form fitting and hugs those shapes....goddamn.
So when I see a girl that has ALL of that going on all at once, it just kills me absolutely dead.
That's the physical stuff. As for more psychologically related stuff, I guess when a girl has some of the same interests as me, especially because of how uncommon some of my interests are, it's also sexy, but in a different way.
But really, I'm thinking more of the physical aspect with this question.