hi godslayer. start with head, i like your style of faces and i also applaud your lack of dithering. is this by choice?
1. colors. wow, you have a big palette there. see how you have quite a few hues with only a couple of variations on each? you might get a better result by having slightly larger ramps of each hue (i find about 4 works well) and less actual difference in hues. This will also allow you to do easier overlaps of colors where needed. the colors of shirt and pants aren't working too well together.
2. tone. there are no major definitions of form with tonal variation, everything is roughly the same tone with slight detailing. this may not be as simple as correcting what you have done, but the problem may lie in the composition/planning stage. poor can be made good, good can be made great. i think the arms receding into the z plane should be shadowed. hair needs work... shape and style is perfect, but looks unfinished, which it probably is. finish it... add general highlights and seperation to give it some 3d form, looks fairly flat at the moment compared to style of face.
3. structure. good. its a pose. i would have put arms in front of body with forearms coming down straighter.
4. general. skin won't usually highlight like that unless oiled up. make the skin the color it would be under whatever light it is under, then shade away from that as the light is lessened in areas. shading defines form, not highlights.
you have a great style in development godslayer, keep it up! (your faces are great)