-I've messed with the shapes a lot, but the main thing that you should change is the arms; make them longer. I changed the design of the armor to something I could visualise better, but you don't need that. I tried to mimic the muscles-on-the armor thing, but I couldn't figure how to get that to work at this size.
-Yours is a bit flat because it's mostly midtone. I think mine has about as much shadow as yours, so mostly you need more hilights.
-I switched the lightsource to top-down because it means you can use the same sprite for facing left/facing right - you wouldn't need one of each. I'm not sure if you even need that or not, I kind of did it on automatic.
-I added outlines, one shade darker than adjacent pixels, with exceptions where it would make things hard to read. This is just so the character pops out from the backgrounds. There's other ways to do that, but this is the easiest.