@Ro83rt:
While your edit helps with the misconception about the neck, I don't think it's
quite on a different plane yet. I think it's because there is still about the same amount of shadow covering the face as the neck, thus creating the illusion of the neck being as close to the camera as the face. The face usually casts a very harsh shadow on the neck.
All in all, though, your edit and critique addresses most of the glaring issues that I saw in the original piece.
@ndrake:
I made a quick, dirty edit to show how I would go about shading this face;
Note: it would be very helpful if you posted your reference picture as well; faces are very diverse.Basically, what I did was try to bring all of the shading together. Your piece has little "islands" of shading that makes it look a little bit odd. What really stuck out to me, though, is that you have no cheeks, or a jaw. Your face just kinda falls into your neck. All you need to do to fix this would be to stretch your cheek highlights down all the way to the bottom of your jaw, and add a much darker shadow all the way along the top of his neck. Of course, if you want to really fix your face up, I think you'll need some plastic surgery, just kidding.