1. Do something with that background. A plain square is just not necessary.
2. You have enough colors to add some AA in the outside and not just the bald part.
3. The lines and shapes in the eyes are not defined enough.
4. It looks like a mix of styles. In some places you use a black line to define forms, on other places they are just dark or simply inexistent. And you use dithering in just one place.
5. The frown line is too dark
I'll do it better in the next one.
I disagree with point #1 (and agree with the other points).