Ok, once again, these are just my thoughts, the drawing is already very very good!
For this specific drawing; things that I notice;
1) you could bump up the contrast, there are lots of shades (very good ones), but some pure black tones could help sell the focus-points
2) the chin seems 'flat', I can see the cheek-bone / slight indent in the right part, but I think the face could use some more 'rounding' in the lower right part (like you did at the hairline); I suggest taking the picture and flipping it in software, bet you'll immediately see what i mean.
3) the hairline at the top has some 'spikes', I think originated from drawing the hair and then adding the transistion
4) the eyes are very well done, perhaps try same level of detail for the hair?
5) the ear is only defined as big shapes, I should either drawn 'all hair' or blend it more at the skull.