A little brush up on anatomy wouldn't go a miss. The pose is very stiff, and the curvature of the arms is way off. Some of the lines are very ugly looking due to the effect of you attempting to create bumps and folds into the clothing, easily made mistake.
Ok first things first. Go back and get some reference. Nothing fancy, maybe someone standing in the position you want your sprite to be like, and similar clothing if possible. Make sure you have the structure of your sprite layed out nice and correctly before moving on.
His arms/hands are very awkward, he looks like he has the fronts of them uncomfortably rotated round, nobody stands like that.
The feet too, are at very awkward positions, there isn't much more I can say than to get references. Make sure you know the angle your looking at the sprite from and remain consistant with that. If it's a straight elevation then remember we're only going to get the very straight view of something, not awkwardly sloping angles.
You show decent, but basic knowledge of shading, and thankfully the pallette you have isn't in crucial need of a change. The black and white visor needs more detail, otherwise it just looks awfully flat. First think of your light source, then the shape of an object, let these factors immitate how you shade the particular object.
Look at the posts made by missingno in this topic, especially his examples and links.
http://rmxp.org/forums/index.php?topic=50879.0
The main task ahead of you is to decide if you want to keep this stiff pose or try again, and once you've decided it'll be time to go back and look at those lines.
