I think this portrait is great for a quick work.
I think the problem with the hair, is that you made it to "strandy" (I cant think of a better word. You need to think of it first as areas of values, then as individual strands, and I think you will get much better results.
Heres a simple tutorial, not great, but the best i could find quickly, that demonstrates what I mean.http://homepages.tesco.net/~p.wilkinson/DrawingHair.htm
Kepp it up though, I think you definetly have a talent.