All the maps posted in this topic have a couple things in common:
1. Sand and gravel don't fall.
2. Water doesn't flow.
I think it's because the maps have been modified somehow to make it so the blocks don't update.
These can be real problems for people who want to build things other than pixel art, especially the problem with the water. It limits people from being able to make some things like moats, pools in their house, irrigated farms, sand traps/sand-falling doors, etc.
You'll also notice that in survival mode, when a creeper dies and blows up, no blocks are destroyed.
If anyone can post a normal flatmap with no modifications to how the blocks behave, that would be excellent.