Well in human nature the natural goal of life is to reproduce and humans look for signs in a potential mate that will get them the most efficient partner they can. It's known that a more attractive person is typically one that has general symmetry to their face. Also in women, usually more feminine features are thought to be attractive, the same for men, more masculine features. At first glance, I personally look for someone who's physically fit, has a good sense of style, good teeth, pretty skin, etc. Just generally attractive features, smaller nose, big eyes, full lips, long hair is a plus. Also the way a girl carries herself is important. I go for a girl who's got it generally going on, confident, but not someone who thinks she's a snob. I will purposely ignore a girl completely if she acts like she walks in and wants everyone's attention. That's a turn off.
I am not sure about other guys, but I first examine their face, then what they are doing and how they are reacting to those around them. Once I have concluded that these are suitable, I examine their overall appearance, and then if everything is still okay I talk to them. If I find myself talking to them before I have a chance to do this, then I will judge their actions first, then their words, then their face, then their appearance.