I'm not good at critique, but i do know it's nicer to earn more of a comment than "cool" or "good job" and it's so refreashing to see an artist actually asking on how to improve. Though, there are some things that can't be taught and the artist has to find for themselves.
That aside, it's been mentioned already, but the light source is a bit difficult to pinpoint. For example the light source does seem to be coming from the top right corner but the shading on the foreward arm continues along regardless of that. And the white colouring on the eyes looks a little too sharp compared to the soft cell shading of the rest of the picture. The line of were the colour lightens on the lower half of the face could be a little more curved to show that it's not a 2D picture.
Can't find any faults with the BG though ^^ That looks nice and smooth. Wish i could do BG's well *sigh* With it being darker it makes the character jump out at you more, too.
Sorry if i sound harsh... i'm really not good at putting thoughts into words ^^; I'd say practice makes perfect, but it doesn't as nothing is perfect and everyone can improve their work -^^- Keep it up, you're a good artist!