Aside from the somewhat chicken leg syndrome people are speaking of, the sprite seems to move too much in the side-views. Like, in the front/back views, he just looks like he's walking, but in the side views, he looks like he's doing an enormous stride, or is in mid-skate motion ...
Additionally, that outline is really harsh. Don't be afraid to shade your outlines! Remember, they only have to be about one shade darker than the fill next to it. They can be darker, but near the light source it needs to ... lighten ... more.