Your colors and lighting are nice, but you need to devote more time into the silhouette and outlines of your sprites. You have a lot of "jaggies" in your outlines, the curves are discontinuous rather than smooth, and lines fluctuate in thickness.
This page has some information about jaggies, and also a section on cleaning up an outline. I think if you made your outlines more carefully and spent more time rounding out your shapes so they are smooth rather than lumpy, you'll notice a huge improvement in all of your art