I was going to explain my advice, but god dammit it's just easier to give examples. I made a bunch of changes in some of the the areas I was going to mention.
First of all, outlines. You should try breaking up the lines sometimes, as it helps to give more of a sense of shape. As an example..
Also, I don't know if it was because the image was compressed, but you seemed to have quite a few colors that had very little variation. That is, a blue would be next to a blue that was so similar that it's difference wasn't visible at a glance. This may not effect much visibly, but it's good to nail down a set of colors and re-use them where you can, rather than coming up with tons of shades on the fly. It helps a lot when you have to color multiple frames of animation, as it keeps things consistent.
Also, color. A good strategy is to give brighter colors more saturation, and give dark colors less.
Anyhow, I need to sleep. Keep up the good work!