Just played through this, really really good! Couple of thoughts:
1. A blob shadow below the sprites would make it easier to gauge depth, and would make some of the jumps easier.
2. There appears to be a bug where gravity keeps accelerating you down when you are ion the floor, so if you don't jump and then walk off a cliff you drop downwards at the speed of light.
Also, one question, is there a shell on the secret level? I wasn't able to find one yet.
Thanks, great job!