This photo is a stark contrast from your other one - in the body shot, the lighting was nice and even, but in this head shot, you have the light coming from an oblique angle that highlights her hair but none of her face. Also, this angle gives no backlight to the hair on the right side of the shot.
Having the key (main) light at an angle that reflects in both the subject's eyes is a very good rule of thumb. Using the sun as a backlight is very useful, as long as you have a reflector to fill the rest of the subject and avoid shadows.
