Hm... Well, I think maybe add a bit more shadow to the part between the lips and the nose, so it doesn't seem like the mouth comes out so far? I'm not too sure, as I've never successfully drawn a person. O.o;;

See, I never took an art class, so I don't really know what wrong with it. I like it the way it is.

I think it looks pretty good, the only thing I think doesn't appear natural is the eyes and mouth. The way my art teacher always taught me with things like this was to measure the distance between, for example, the tip of his nose the the little arch on the top lip with your fingertip and the pencil point. And then look at somebody else, or if your looking at a photo the photo, and see the difference between how it is and how it should be. After a while your eyes are trained to little things like that. That's how I was taught, maybe it'll help. But other than that, I think it's a really awesome picute! The shading in it is great!