I don't see why you're dithering so much when you've got 26 colors to work with. I've learned the hard way a couple months ago, that there's no point to add a color if you're never going to have it be solid anywhere. I can see where maybe you want to use if for color blending purposes, but you could accomplish the same thing by just picking out the color.
The shading itself, though, is good. You could maybe use a little work on the hood, but otherwise it's good.
Just wondering if you freehanded or traced?
Bottomline: the piece is not bad, and I especially love the background, but if you're going to use so much dithering, why not try to restrict the amount of colors your using to 16 or something?
(sorry if this is a poor critique...i'm pretty tired...)