Minecraft wiki entry for Development_versions, associated text: Biomes are now much bigger and more flat.
Hills and mountains tend not to generate as much, except in the new extreme hills biome.
They are no longer aligned based on temperature and humidity, instead their distribution is random.
Swamplands
They have been renamed to just "swamps".
They are a much more vibrant green color.
They have a new tree type, which are slightly bigger and have vines hanging from their leaves. They can also generate in shallow water pools.
They are much more flat.
Taiga
They no longer have snow nor ice.