I totally agree with your points, rulric. I found myself a bit conflicted during voting because I did not want to discourage devs, but I also did not want to inflate their ratings in a way that felt dishonest or out of pity (for the sake of not discrediting someone else's game that is strictly more "Tamagotchi-like"). A third category like "Game Completeness" (or something similar) would do a great job of providing devs additional feedback while not discrediting submissions that better adhere to the intended submission rules/types.
I've never hosted a jam before, so I'm not sure what's capable with Itch's system, but it'd be cool if each category (Gameplay, Art, Audio) had ratings simply for the sake of feedback (even if they were not factored into which game actually "won" the jam).
EDIT: I read Lyz's comments (and plans for 3 category honorable mentions) - very cool! XD