Oh god, this is awesome. I especially love the far background....so yummy.
My main concerns are, first of all, the the splash of water from where the fish is jumping out; looks out of place, an not much like a water splash...donno how you'd fix it, but it definitely stands out. Secondly, the ladder, (though it may be due to some sort of tile restriction or something) just goes into the floor... from what I can remember of the platformers I've played, the ladder goes up to, or over the level of the floor (usually without the floor behind it, but sometimes...).
And then, just out of curiosity, what is the fuzzy orange stuff...tree mold? I'm not saying it looks bad or anything, but what's it supposed to be?
Either way, this is a mock-up. Great work.