It only took 127 attempts and some liberal bonesawing, but I finally beat the last boss. It's a really fantastic game :)
If it's helpful, there's a few minor gripes I experienced but please set this against the massive amount of enjoyment already in the game:
* In the first world and third world particularly, it's annoying when you walk into a room and immediately take damage from a ball-and-chain or pop-out spike, especially if you're quickly back-tracking through rooms you've already cleared. I wonder if you could do some code-magic in certain circumstances so that it's impossible to enter a room and hit one. I guess I should've been more careful though, so it's not exactly unfair :)
* If it doesn't affect balance too much, could the game lean into the player's playstyle a little more? If I'm primarily using a melee weapon, it's a bit of a bummer to continually get ammo boxes out of crates. It's not that they should never appear, just that the weighting could be dialled up and down depending on whether they're going to be used.
* Shops with one to two items in them are a bummer, especially early on.
* Sometimes the bolt-sink (the postery thing that says 'Done T' in shops?) doesn't give you anything, even if you fill it to completion. I think this happens on later floors when you've already got some items from one on an earlier floor.
* I wish the Duck buff triggered immediately on entering the boss room. There have been a few times where I've died before getting any health back.
* If you attempt to pick up an item that overlaps the Sell box in a shop, it gets sold instead — I think. It's only happened once but it stopped me duplicating all my passives :(
* The jump pads in the third world feel hard to use consistently. I seem to often take damage from adjacent lava, and doing diagonal jumps in enclosed spaces is also tricky.