Well, the first thing I look at is the face, not the body. I like dark hair, and semi-long. As in, Jim Sturgess in Across the Universe long. Long enough to play with, but not so long he could put it in a ponytail or something. I'm extremely drawn to eyes, and this is pretty much a make-or-break factor for whether I consider a guy "hot." It's not an easy thing to tie down, but when I look at his eyes, they have to hold me for a second. If there's a certain intensity or confidence there, I'm hooked. And, as Jengirl said, smiles are extremely sexy.
Body-wise, I'm told I'm weird in this, but I go for two different body types: the military look with strong arms and upper body, and tall, skinny guys. The former because I think of him holding me, and the latter because I just find it cute. Being tall is fairly important, mainly because I'm tall and I would feel odd with a guy who's more than about an inch or two shorter than me. Obviously, none of this is a must; personality is more important to me, and if I fell for a guy with none of these characteristics, so be it, I'd still date him. But these are what initially attracts me.