construction - basic shapes, directional lines, angle, etc

expression - giving the face a clear expression and sense of 'life'

facial features - getting the eyes, eyebrows, nose and/or mouth to look right

volume - giving the face a 3 dimensional feel, giving it depth

skin - getting skintones right, giving depth and life to the appearance of the skin

variety - drawing different types of faces and features

stylization - applying a certain style (cartoony, painterly, anime, something totally unique etc) to the face, for an interesting end result

realism - getting the face to look 'accurate' and 'realistic'

(almost) all of the above