Very fun. The attacking mechanics were smooth and enjoyable, particularly the "down air" attack. Ijust used it at any chance I got. Although I'm curious as to why it wasn't explicitly mentioned, as I only found the move out by chance.
The boss fights were great - they came unexpectedly and had nice twists to them.
I like the dynamic within levels where a player could choose to aim for the secret, at a cost of difficulty. Secrets are nicely placed as well (the crumble in the wall is a great, since there are some times where I actually missed some). One comment is that it was annoying to try and blow up crumbled blocks that were placed horizontally or underground, since you had to gauge the range of the bomb.
I'd like if you were able to use a bomb while in air, just to provide more options and control to the player.
I like how dying from mising a jump only loses you some HP, rather than conventional games where the player would have to restart the level.
All in all, I see a lot of the mechanics are inspired from classic games, yet the game itself has its own twist and mechanics which makes it its own entity. Great job on this one :D.