the main thing to me are the color ramps, they don't depict the materials properties.
For example the skin will be more saturated and red while being in the shadow than fabrics, fabrics won't have as bright highlights as metal and so on.
Cast shadows should be darker, like under his hat for example.
Also having the character foot cut by the frame make him floaty.
I did a quick edit, not being a pixel artist it's quite horrible shape wise hehe.
this
tutorial should make things easier to understand (sorry for my frenglish)