I am always trying to explain to people what I mean when I say I'm attracted to "butch" women. That's the best word I have to describe it - that particular characteristic I fall for, the strength, self-assurance, aggressiveness that comes with restraint - it's not just short hair or body type or self-confidence.
This article at After Ellen, about Rachel Maddow, gives the best description I've seen yet:
The set of a woman’s shoulders, the way she crosses her legs, the angle of her hips when she walks - all of these are signals of butchness. And we know it when we see it. That kind of recognition takes place in the gut.