It works because it has a consistent colour scheme. I believe any attempt at "restrictions" would get the opposite reaction, personnaly nsfw contributions don't stop me adding tiles in anyway nor do I care one way or the other (each to thier own). - If anons contribute thats great regardless of skill or content.
What might develop or enhance the game is an established colour scheme, a consistent angle such as top down 90' iso/angled or side scroller. This loosley encouagred if anything.
Working together is great for aethetic coheasion but I'd never prefer it enforced (I join ths type of thread for the casual art interaction amoungst Anons and on 4chan because of it's eclectic platform).
I felt this after adding F7 which contains a sky (now I realise a poor descision to the surrounding tiles).
Balance between free for all fun and a positive honour system amoungst Anons.
If an Anon feels they cannot contribute due to the map being too narrow in scope then the spirit of the whole thing has lost something potentially.
Tile map general threads for all flavours maybe : D.
Great work so far nearly finished, I look forwards to the next one.