It's a good start you have here. You've tried to get some subtle shading around the face, especially around the eyes that adds to a realistic look. The position of the face, the 3/4 pose, is more difficult to do...and more effective than full on.
Some suggestions though. While your eyes are good with the thick and thin line around them, this is one of the few places an outline should be used. NEVER put an outline around the mouth as it flattens it and makes it look like he is wearing lip liner. Use only shading to create the 3-D ridges and curves. The only place where a varying line is appropriate is on the seam. This goes for the philtrum (the groove above the lip). This should have subtle value, not be over emphasized.
The same goes for the nose. Keep the hard lines to the nostrils and slightly sound the curve of them. The rest is shading. If you have any lines running up the nose, it makes it look like a snout.
Another suggestion is the placement of the nose and mouth. While your features are in the "classical" positions taught in art classes, I find most faces rarely follow the ideal. Shorten the distance between the nose and the eyes, as well as the mouth and the nose. Right now they are a bit far apart, especially the nose to mouth ratio.
Lastly, try to pick out some highlights using a kneaded eraser on the fur. Shading is good, but having a good sense of the key lights will make the dark areas that much more effective.
As I said before, you show a lot of skill in your draftsmanship. Portraiture, especially fanart with out a reference photo, is tough. Keep on practicing as I'd love to see more!