k, I am going to attempt to critique this piece, despite the fact that is is a very strong sketch, and a great representation of Dr. Strange.
Above all, I think its important to not that there is a lack of anatomical harmony. There are parts of his anatomy that look very long, and parts that look somewhat squat. his arms are very long, his midsection is squat. To the point where it makes it seem as though his fingers would actually meet his knees. Which is not necessarily a bad thing, but in this case, does not harmonize with the overall image, which seems to favor a degree of realism.
In the face, I think you neglected to evaluate the spacial relationships that would validate a face like Dr. Strange's. The jaw and chin are incredibly dominant, and the nose and brow are very subdued. I suppose its more an issue of personal preference, but I've always liked the Dr. Strange with a pointy nose, and a prominent brow.
Most importantly is the issue of the legs. This is more a critique on rendering in comics in general. I know that this area of his body would be black, but by having no other black on the figure, you draw far too much attention to this area. Its important to remember that regardless of an areas value, if the scene is bright, the object will not be solid black, visa versa. If you want his leggings to show that much true black, then there should be several other areas of shadow in true black. Harmony in your piece is very important, and you cant rely on a color rendering to solve these problems.
Anyway.. thats my input. Hopefully its more helpful than offensive. I do love the overall image.