http://www.mojang.com/2012/02/14/new…p-format-anvil/
- Maximum build height has been increased to 256 ( was 128 )
 - Empty sections of the world are not loaded into memory
 - Block ids have been increased to 4096 (was 256) by adding a 4 bit data layer (similar to how meta data is stored)
 - Block ordering have been changed from x/z/y to y/z/x in order to improve compression
 - Packets for sending chunks have been updated (a full 128-high chunk 
is smaller than the old format, and a chunk with lots of empty space is
much smaller) - Biomes are saved per x/z column, which means they can be altered by tools
 
and more...