Great stuff! I sometimes had difficulty identifying what was able to be climbed, as the foreground often went over the midground and was sometimes the same colour as the ground. The mushrooms were very hard to identify, since they had an almost identical colour to the background. Another thing was the spikes that burst out of the floor (in the level and boss fight) didn't really have much warning.
I really loved the game though, looks frickin incredible. I hope these suggestions can make it even better!