The text at the start is somewhat difficult to read due to the font, and some of the sprites on the "Island Map" don't fit with each other. The object at The beach is extremely lacking in pixel count, and the cave is too detailed. If you move under a cloud platform it can push you under the ground (to some degree). The amount of coyote time you have is a little bit too much. You can double jump infinitely, regardless of how many times you've jumped mid-air.
You can sit over the ledge on the bricks without falling. I think the hitbox is off
As I previously mentioned, you can jump infinitely, allowing you to go high enough into the sky to build enough velocity to clip through the ground and fall below.
You should also add checkpoints and enemies, otherwise it's a very nice game with lots of potential.