For men: First thing is probably their general dimensions: how massive are they, and if they are large, is it height, fat, or muscle, or some combination. After that, I notice whether they have class, swagger, or both. Do they look confused or composed. Things like that.
For women: There is an instant desirability assessment, along the lines of “do I want to know more.” I’m a sucker for a pretty face (for me, a pretty face can make up for many body imperfections, but not vice versa). I like all the other body parts that have been mentioned here, though after the face, I tend to be more of an A guy than a T guy. The other thing I notice right up is whether they seem happy, angry, sad, stressed, confused, or bored. And when I’m talking with her, I’ll notice whether she looks me in the eyes or not.
Yes, I agree. We may not notice good posture consciously, but I suspect we perceive it and react to it anyway, since good posture tends to connote confidence and positive attitude.
And yes, the way a woman walks is very noticeable. Some march, some sashay…