Here's what I see:
You're using black as your background color, which can throw off your eyes. Try using mid-tones for the background.
You're using the background color (black) in several spots on the figure. This may cause you problems with transparency later.
I see two choices that I don't understand. They show up in other people's work too. I find their persistence rather strange.
1) Long torso, short legs. I could see having a long torso if the character is fat or pregnant, but this character is neither. Legs generally take up half of the body length. Since his pants are not skin-tight you could drop the crotch a pixel or two lower than half-way, but the legs here are just too short, and the torso too long. Or maybe my sense of proportion is off because I see this a lot.
2) Tiger-stripe wrinkles. This one I just don't understand at all. I get that the shirt should wrinkle, but the horizontal dark stripes don't work for me. They're not rips, but they don't follow the drape of cloth either. Here's a tutorial on draping cloth and clothing:
http://conceptart.org/forums/showthread.php?t=14739 I haven't done any of this with pixels yet, but it should work.
Other crits:
The face isn't very clear. This is a profile facing to our right, correct? I think the neck should be entirely shadowed to help show where the jawline is. Maybe darken one pixel by the eye to suggest a bridge of the nose. Maybe one more pixel width to the back of the skull. These would help suggest the shape of the head better I think.
The artificial arm is cool. No crits on that.
The tan leg is forward, the blue leg is back, but the shoulders are opposite of this (the artificial arm is forward), and the head is turned hard to the side. When I try to stand like this I find that most of my weight would rest on the blue leg. This figure looks like more weight is on the tan leg. Try it yourself, maybe I've just got poor balance.
No comments on the colors, other people around here are much better at colors than I am.
Hope this helps,
Tourist