Oh wow, I can't believe that I haven't come across this yet!
I really love how dark the iris/pupil of the eye is, it give it a really strong, striking effect - and the eyebrows are very well done. Also the shape and shading on the lips are flawless.
Another factor that makes this portrait amazing is the hair. It's obvious that you've put a lot of effort into it, and it looks amazing. The little detail of the outer frizz is just so pleasing to look at.
Sorry for the really long comment, but it's been quite some time since I've seen a pencil/graphite portrait that was so well-executed. ♥ ♥ ♥