I encountered a bug in which objects on the conveyor belt would disappear, sometimes causing the game to automatically give me a letter (I think this only happened with bricks, but I may be mistaken). There's also a point around the 30 second mark in level 3 in which two bombs and a trash item spawn simultaneously, which basically guarantees you gain 2-3 letters at once. That was a hard wall I couldn't get past.
I don't believe the diversifiers were used to their fullest potential/properly. I don't think this game counts for Shapely, according to the rules for 3D games on the jam page. In a Song may be properly used, but I honestly didn't notice while playing because the game doesn't really do anything with the concept other than having background music. Mouse controls kind of contradict the point of Two Button, because a mouse allows the player to make 360 degrees worth of inputs at any given time.
I assumed your game was using the concept of checkpoints in a "warehouse sorting checkpoint" sense, but after reading the description on your game page, I think the implementation misses the point of video game checkpoints. Having multiple levels isn't really the same thing as having checkpoints, game design-wise.
All that being said, I enjoyed the physics of the trash items. Picking up trash and chucking them into their bins instead of dragging them was really satisfying. And although I don't think the game's visuals count for the Shapely diversifier, I did like the graphics in general. Everything looked nice and polished.