I think the only thing that I see that really doesn't look quite right is the mouth. It's much to planer, when it should be more cylindrical. Here's a tip that I used when learning to draw the mouth, but still applies to 3D. Try imagining that the mouth you are modeling is wrapped aroung the front of a soda can. Notice that the mouth would have to follow a curve, which would make it more round.

Here's a suggestion to do this in Maya. Go to nurbs primatives, and create a cylinder. Go to the front viw and scale the cylinder roughly to the height of the head. Then scale the cylinder again to match the curvature of the model's face you have alread made. Move the cylinder to where you feel the mouth needs to be, and start pulling your vertices to match the curvature of the cylinder. You can either make the Cylinder a template or assign it to a layer and switch the layer to reference. That way you don't accidentally select the cylinder all the time. Hope this helps.

Good luck with your modeling.

Chad