Rated!
This a great little game that grows on you!
I really liked the way levels transition, and the tileset gets higher res. Great touch.
I agree with the other poster, if the sword did more damage the bigger it is it would feel a little better. Just being able to one hit those enemies you've outgrown right before the end of the level.
Minor nitpick: "You can now double jump!" In the future that might be better if it was like a new different colored mushroom. It just feels slightly better to actually pickup a powerup, than to be granted one at the start of the level, imo. Just a small thing though.