Faces are beautiful based on a mathematical formula of symmetry. If you were to draw a line down the centre of someone's face and the two halves are equal-ish, then you have what is scientifically considered beautiful. It's what the person lurking behind the face possesses that transforms them from beautiful (or cute, as Luke suggested) to hot.
Hot is an attitude, an essence- it's something you perceive about a person and you feel it as your body makes the call. You react physically to them, which is hot in and of itself.