First things first, the overall texture and color of the piece is so appealing, and the line work is great. Good job!
The reason I faved this one as opposed to so many others that have a similar subject is that it looks like mixed media, or she looks like she's suspended between the world of photorealistic art and the world of sketches.
I love that her skin has more detail and a more lifelike color, while her hair is much more stylized. Interestingly enough, the same splotchy texture which seems unrealistic on the hair reminds me of freckles when it's on her skin. Because the hair is so boldly stylized, it frames her face and moves my eyes right to it.
When looking at her skin, I want more detail. This piece is wonderful, but you might try adding detail (like deeper shading) on her body to make the contrast between the skin and hair stand out even more.
As for composition, the arm feels a bit big. Also, it pops out of the frame since it has less deep shadow than the face, which recedes. If you added more shadow on the arm it might recede a bit more (I think...)