I had a realization: pretty eyes are open eyes.
Not just that they are not closed... but if you notice, most people's eyelids cover a lot of the iris. With eyes that we consider beautiful, that's usually not the case, almost all of the iris is visible. It creates an impression of awareness, and a more aesthetic shape of the eye, I think.
This of course is very different from eyes which are more open than their relaxed state, such as in fear, I'm talking about the relaxed, natural state.
Update, a helpful anon found this video: