Your biggest problem besides anatomy is that you're using pillow shading. Like yaomon17 said, where's the light source? All of your segments are dark around the edges and light in the center. Even if the light were coming straight on (going into the monitor) it wouldn't be so uniform. I would suggest just starting with a smaller piece, say just a bust of the cyclops; you then have less area to finish (and thus might not lose patience, as you say). Pick a single light source (top or top-left are always good picks--we have it in our brain that light usually comes from above), and define at most 5 colors for skin variations.
Here's a good article about shading by Derek Yu, which you should definitely give a read through: http://www.derekyu.com/?page_id=227
@Hatch:yes maybe a little to ambitious, yes I should have spent more time on the sketch before coloring the whole thing.This was a good practice, maybe I will do more later.
This is certainly ambitious! From the looks of this, I'd say your approaching this in a manner that's only likely to frustrate you in the long run. You should rough out the shape of the body with very sloppy 'blobs' before you get detailed. Do many many many before you settle on one to finish, and really try to nail the anatomy before you even think about individual pixels. Meticulously constructing the human body from the pixels up like you've done here is really just gonna be exhausting and ultimately less effective.
Again: loose, rough sketches -> refine -> refine -> refine -> pixel level detail
I wanted it to look like the lightsource is on the top right side but I didnt make it obvious enough.I need to practice shading.