You wanted to learn how to sprite, so you chose the laziest and most boring character design possible. Seriously, it looks lazier than a crawla's design.
First off, the color. Its too dark, and has little contrast, thus making it lack definition and volume. The fact the you constantly shaded it horizontally instead of using the shading to define the figure only hurts this sprite more, you couldn't just shade a circle right? (What's the point of overly shading the hands and feet anyway? They're so small and hardly noticeable in comparison with the rest).
And For a ball with stick legs, the linearts also bretty wonky/wobbly, and the light-source's all over the place.