Minecraft wiki entry for Caves_%26_Cliffs, associated text: Terrain generation is now independent from biomes, with biomes adapting to whatever terrain they happen to generate throughout.[14]
This allows for natural variation in the terrain of any individual biome, both in terms of shape and elevation.
For example, forests and deserts could form up on a hill without needing a special biome just for that purpose.
This additionally allows for smoother transitions between biomes.
Terrain is now smoother and more extreme.