Kinda what Extrude said, except a little more specific. The socket area for the eyes could use a little more shading to show that there are actually eyes in there, but not necessarily any more specific lines. Proportions are just fine except for a couple little things; his right ear, and his right arm. They stick out in a way that's not really physically possible. Shading, I'm actually fine with except for on the eyes (and possibly on the hat).
Oh, and his neck looks a little fat.
Overall, those are little tiny nitpicky things and are not too important. Nice work!