A combo of the face expressions, mannerisms and opportunity.
With opportunity (as far as I am concerned) there a millions of pretty girls in the world but you walk down the street and see a pretty girl you don't run after her and shout for her number. So it's more if your by the bar, outside in the smoking area, in the lift etc. and a pretty girls next you. It's about visibility you covert what you see, and you see people in mutual places where it's more aimless you take time to look around and take more notice.
eyes. Well, technically the face, but the highlight of the face that plays the "make it or break it" factor on looks is the eyes. I just like vibrant, alive eyes. Like that one random person said, the eyes are the window to the soul. But then obviously the personality plays a role after I get to know the person