I really love this piece of artwork, I think that you have done very well with creating a very realistic portrait. The thumbnail instantly caught my attention, the contrast between the dark and light shades really make this piece eye-catching.
I think that you have created the impression of realism by adding lots of effective highlights and shading. This has been done very well, as you have added the highlights where the light would naturally hit the face - and the same with the shading.
I also love how the contrast between the light and dark areas is very bold, as this creates a very striking image. I know that many artists were very subtle shading, but the technique that you have used is different and impressive.
You have done the hair very well, by adding different shades, which gives the impression of different strands of air. Overall, I love the image, it has been done extremely well. The shading is excellent and your techniques are very original.
Hmm, if I had to change one thing about this piece, it would be the background. This is because I think that it is too dark when compared to the hair. In order to make the shading on the hair stand out, I think that you should use a light background. Though, this is a minor critique and isn't completely necessary, as the image looks amazing as it is.