Portrait Test

Saw this on reddit, though I'd comment here instead.  Like someone in the reddit post said, even though it's stylized it wouldn't hurt to add some additional shading.  Just a bit on the bottom of the nose and the chin/jawline.  There's highlights and midtones but not alot of shadows.  This could just be the style you want but having atleast 3 different shades can really make something pop more.  Try using flatter,harder brushes for shadows (similar to how you did the hair), you don't want the skin to have too soft shading.  My only other thing would be to maybe add a bit more saturation or highlights to the hair?  It looks good but feels like it's missing something.  Experiment with color colors in the face, usually there's subtle tones of red in the face.   When shading lips, a good tip to remember is that in 95% of cases the upper lip will be darker than the bottom lip.  Just making the upper lip more shaded can help the form of your painting.

I'm still trying to get a hang of digital painting and everyone has their own techniques and style but here's a quick run through on how I would add onto your picture:

i.imgur.com/aO2kfqM.jpg

Great art, keep at it!