As for your head, it just looks too much like a block at the moment. The main areas that are bothering me are the eyes and the mouth.
The eyes just aren't the right shape. At the moment the outline of the eye looks too oval. There should be a flat part at the tear duct on the bottom edge. This area should be fairly low compared to the outside corner which should be higher. The bottom edge should have a slight 's' shape to it. Also the high point of the eye shape should not necessarily be in the middle. You should really model the eye around a sphere to get a good a shape. The eyes are also looking a little high - they should be half way up the face.
The mouth is probably the weakest area. The space between the mouth and the nose (the philtrum) is just too small. Also the corners of the mouth are looking pretty weak. Basically the lower lip should disappear under the upper lip. I would also bring the lips out more and also pull the middle of the upper lip out a bit.
The forehead also needs some work. I would get rid of the angry lines that are coming out of the top of nose and then focus on the transition from the brow and the top of the forehead. At the moment it looks like the forehead is just too slanted and sloping back.
Obviously, characteristics change from person to person, so nothing is written in stone. If you are not already, you should try working from a reference image.
Hope this helps. Keep up the good work.
