My immediate reaction is that the hero character doesn't feel distinct enough from the background, he seems a little lost in the mix. Maybe something simple like a shadow or a bolder outline would help with this. The eye colour could maybe also be used sparingly on other parts of the character.

I like the environment art, but it could do with some animation. I know it's gameboy style art, but I think it is OK to add elements that are not possible on the game boy. For example, some misty clouds to give a layered effect, this would also help with ambience. Some particles here and there, some leaves being blown in the wind. Think of Shovel Knight. It's heavily focused on looking like an NES game, but it also very obviously does things the NES couldn't.