the 2 major problems with this is that everythings skewed, I know having things exactly symetrical isn't good but you gotta make sure that there is some stability in the general proportions. atm the right side (our left) of the head is much larger. this is a quick demonstration of how I think it should look, I just flipped it over to give the head a better shape.
the other major problem I see is that you are dithering needlessly, unless your keeping to a certain palette or limited number of colours I wouldn't go so mad dithering like that. it's just giving this a horrible textured look. imo it looks very messy. It would make more sense just to add in that extra shade between the lightest blue and second lightest.
I'd also suggest cleaning up some of the linework in places, adding a tad more AA.. it just looks very random imo, I always like to plan and take things one step at a time, otherwise you just jump into something and spend most of your time altering it to what you originally should of considered.
trying to edit this to illustrate how I think it should look.
help at all?