Quick disclaimer: I'm pretty new to all this and just learning myself. There's some good work here, you should be happy with this as a first draft!
Given that, a quick few points:
1: I love the shapes you have here, but it looks like you've taken almost every shape and just shaded from one side to the other. This is forcing unnatural angles for any given light source. Pick a light source direction and imagine which bits would be bright and which in shadow. I compare this style to pillow shading, where a standard pattern is applied to all shading areas regardless of light source, angle of component, etc. There are a few places where you've added in shadows, think about where else in the figure shadows would occur, and how shading will flow naturally around shapes.
2: The rear leg and arm may look better if they're one shade darker across the board. This will give z-axis information and provide more depth. The same for the rear side of the front of the coat, if you see what I mean.
3: The tops of the boots and the left side of the buttons down the middle are crying out for some shadows...
4: There are some jaggies in the outlines that could be tidied up. IMO outlines are best kept thin, one pixel thick, without clumping or steps at the diagonals. As they don't convey any actual information, another option is to just go round the edge and create an "inside outline" by using a darker shade (one value darker for each pixel) for all edge pixels. But it's a matter of style and preference.
5: The arms are a touch short, especially his right arm (our left). If you imagine straightening out his left arm, it will extend just past the waist. His right arm barely gets there. If you stand up and relax your arms, you'll probably see that they extend a little way into your thighs.
6: Shading again. Instead of taking roughly equal "bands" of shade and roughing them up with noise, have the bands occur very near each other with large areas of the same shade. thus you reduce "banding" and create more realism. If you look at how things are shaded around curves in reality, you'll probably see what looks like a sharp cut-off of shade from bright to dark across the face of the object.
7: Everything looks like the same material. Matt and dull. Pick some bright colors and add specular highlights to shiny objects. What is the jacket made of? Shiny leather? Matt suede? Are the boots polished to a shine? Is anything made of metal? Anything that's shiny should have point or near-point highlights to represent shininess. Larger areas of dull highlights (as seen across this image) convey matt surfaces like skin, wool, etc.
8: The only place where the outline of the coat reflects the crumpled nature of material is the lower part of his left arm. The rest shows straight, smooth edges but contains noise attempting to suggest creases and folds. This confuses my brain somewhat. I'd like to see what happened with fewer folds, that match up to the edge disruption.
Phew! This is probably a lot to take in. I may have time to do a quick edit in a few hours, to demonstrate some of what I'm talking about. In the meantime, ask me about anything that I've been unclear about and I'll at least attempt to back it up with examples of other work!
Edited to add:
When I started playing with this image, I noticed something. All the lines seem to be pointing down.
- The hat is pointing to the ground.
- The face is looking down.
- The shoulders are rounded and hunched.
- The weapons are angled downwards.
- The arms are hanging a little limply.
I'm guessing you want to get across a fierce, dynamic character. Some things you could try:
- Separate his right arm from the body to show he's putting some effort into the pose.
- Square off the shoulders, thrust out the chest, pull the chin up, maybe add some neck height. The coat can pinch in the middle a little to emphasize the big chest and the flare of the coat lower down.
- Try a more dynamic use of the gun. You could point it at an imagined enemy or even at the viewer. With the former we would be able to see more gun detail.