Obviously, really cool art. The cut scenes go on a bit long but did give me a few chuckles. The gameplay feels smooth. I like the kickback from shooting it adds some extra interest to the game-loop.
Some projectiles are kinda hard to see at the corner of your eye. Pair that with not much of an affect when you take damage and its not always obvious when I lose a heart. On that front as well, there isn't really a sound for when I actually hit the boss (although the shaking of the health bar partially makes up for that).
Boss design was quite nice. Felt well paced for a game jam. The variation of projectiles was neat and the visual designs/concepts were cool. I laughed when I realized I was dodging equations.
All in all, Bucket is the best.