Agreed that the arms look weird with the inaccurate muscle anatomy, and the upper body is too long for the legs (or the legs are too short for the upper body... either or). Looking at your sketch, I think you deviated a bit. I also liked the direct look at the camera in the sketch more than the current finished product.
The shadows are spastic, and do not follow a point of reference for the lighting. Try drawing a temporary lightsource to help with placement of shadows.
A dark, shadowy environment would complete this. I am not feeling the gradiant background. I'm thinking of the inside of a decrepit building with a single lightbulb floating above the character... but that is just me...
Great job though. Don't mind the nitpicking, I just give my honest thoughts.