I have some issues with the game:
1)Some tree branches are walkable, but it's not indicated graphically. If I were you, I'd make them more distinct, like give them an outline or something.
2)There's a short delay before you start double jumping, which feels awkward.
3)It makes no sense that the guy with a helmet can only be defeated by goomba-stomping, cause the helmet is supposed to protect you from things falling onto you from above.
4)When you restart the game, it stutters for several seconds before restarting.
Other than that, I think you can make it into a decent game if you put in a little more time, cause right now it's just too short.