Is this just a practice mockup or will this thing really be coded?
Where did the palette come from?
Is the drainage pipe creating that whole waterfall?
The sky is distracting to me - odd color, too light, the skyline/horizon against sky creates too much contrast.
Are you following NES display restrictions or something? Your ground tiles, especially the dirt could perhaps use some enrichment, like a highlight color and/or some AA.
The user character seems rather generic, lacking individuality. All that metallic armor, I'd like to see him walk that tight-rope. Also, his grey form gets lost against that grey background.
I hope those points help you think more into your design.