If they look nice (which means something different to everyone, it might be their cute cheeks, or their smile, or the way they wear a hair grip...)... just something to spark the imagination "what if..."
And then it's all about the personality. Quirky / witty / intelligent / shy and so-on. and usually everyone is some combination of all of these.
This is a hard answer, because as guys we don't usually concentrate at one thing at a time. When we look at a girl, we look at her whole body overall. We like to get a feel for who the girl is. So we look at the whole body first. And then we go on to look at the girl and her friends, and we figure out what type of person she is. We watch her interactions with people. We try to understand her. Next we talk to her to get to know her, and then we get to be on a personal level. This is how guys look at a girl when they are first interested. And it can be any order of those things being first.