If you're going to shade something the contrast needs to be higher otherwise you might as well not bother.

im not 100% sure because I can't zoom here but there might be a bit of banding aroud the chin area, although I think the one thing that would be quickest and easiest to add if you wanted to make changes would be to consider AA (searching for tutorials and guides either in the forums here or just by general googling will be better than I can explain it) to help smooth out the high contrasting areas in particular (such as the black eyes would be appear rounder against the light brown).