That dithering on the tree leaves is bothersome, it doesn't look like leaves, it's almost reminiscent of the MS spray can...Almost.
The shading on the trunk of the tree makes it look like it's made of flat polygons at angles. Like it's made of a 6 sided cylinder or something.
Try some higher value colors..highlights.
Also the general shape of the tree leaves just looks ...weird..like theres a big random chunk sticking out at us in the very middle
The main character is the best, but the face and the arm bother me. He sort-of has a nose, but he doesn't (maybe it's a style thing, but I just don't like it.), and perhaps you could move the eyes up a pixel. The shading on the arm looks ok, but it could be better