Minecraft wiki entry for Development, associated text: Additions
Bedrock
Unobtainable to the player.
Cannot be broken, placed against or otherwise selected.
Surrounds the map on all four sides two blocks below ocean level, and also the bottom of the world.
Water
It was originally opaque, rendered as a full block and on the same layer as all other blocks (meaning it had shadows).
The player can swim in it, but mobs move under it at full speed. Swimming underwater was fast at first, as seen in the video.
Spreads from the world border on world generation to fill any empty space next to it.
World border
An infinite ocean now surrounds the map, making it impossible to fall off the world.
Pressing F now cycles between four levels of render distance.
Pressing N generates a new level, but without a level generation screen. Human mobs also don't get reset on level reset.
Changes
New terrain generator
Terrain is more messy, with more bumps visible on the surface.
Terrain now generates in islands.
No blocks can generate on ocean level, the terrain is always one block above it.
Dirt with access to sunlight will not become grass upon world generation if below ocean level.