Looks quite nice! I think my favorite part of the ground tileset is the texture of the dirt. It's not very easy to pull off something that doesn't look obvioulsy tiled, and you've done a fairly decent job at that. I like how the dots don't contrast more than they have too with this palette as well. I think the edges of the land are a bit too square though.
I think the pipes and the yellow bridge thingy look really cool. I'm not big on the hills, trees and other grass elements though. Their shading is pretty bland and boring right now.
The SMB3 boxes should have diagonal shading in the top right and bottom left edges, I think.