The proportions are rather skewwy. I did a very fast edit to show you roughly what I'd change:
Upped the contrast, axed one of the skin shades, combined the two darkest colors, added some simple hue variation to the skin(yellow for the highlights, red for the shadows).
Added some slight shading to the hair.
Shortened the outer arm, shrank both the fists, lengthened the legs quite a bit and biggified the feet.
Tried to define the biggest muscle groups. (Still working on my own anatomy, so I wouldn't trust me too far with this part)
Simplified the pants a bunch and added a few wrinkles.
Diddled with the outlines a bit, they're lighter on the lightsource side now.
Pushed the hips forward a little and pulled his left foot out slightly, trying to get his center of gravity centered.
This monitor is a bit fuzzy and so really bad for pixel-art, so I wouldn't suggest looking too closely at what I've done, just trying to help steer you in the right direction.