A few suggestions and observations:

• Desaturate all your colors, especially the green! Everything is very very intense and bright.
• Get rid of the magenta background
• Don't dither! There's no need on such a small piece. It just makes it look rough and gritty, like sandpaper, ya know?
• His hat doesn't look firmly seated on his head, like it's floating a few inches above him. Try to bring it down and match his perspective.
• His body is very poorly defined. Spend a bit more time tightening up his shapes and giving him a more certain stance. Don't be afraid to use abrupt angles to define joints like elbows.
• Try to think of the face more as a complex single unit than as distinct individual features.
• Try to eliminate all internal black outlines. Things in real life aren't defined by black lines, they're defined by shading.