Almost there ^.^
In most respect its quite a good picture, a great idea and well executed.
Although the hair needs to be more defined. Think about the weight of the lines - thin, thick, narrow, wide etc because it seems like the darkest tones is the lineart for the eyes and the head/neck. So do the same sorta tone for the shadows in the hair, so it looks more refined. Plus the light reflecting off the hair is usually zig zagged slightly rather than a straight edge....but thats just typically speaking.
I have to disagree with the comment below by Hoboweasel, the mouth doesnt need to be any larger, but i would suggest have a slightly narrower shadow under the bottom lip. A nose wouldnt do that much difference either. It seems to work pretty well without. Well done, keep it up =D