Hah, cool watermark.
It's overall an excellent piece, but a few things i'd change, are the white 'hair', (Or whatever fur it is, i don't really know what to call it.), and the grey part. As Leonardo said, it lacks a bit in depth, to me it looks like that light hair part lacks it mostly.
Grey area has that nice shapes and layer'ish look aswell, but it looks boring compared to those nice details on the mask. Wich is of course understandable, as front is the most important part and almost always spent most time with, but if you feel like you could add something in the grey part, that'd take this head to a higher level.
Anyway, really nice job dude, i actually remember when you made simple cartoonish heads, and look at you now.
Keep up the good work.