Minecraft wiki entry for Pretty_Scary_Update, associated text: New debug features.
Superflat world is now customizable.
Changed portal mechanics.
New NBT tags to spawn mobs.
Boss mobs now show their name above their health bar.
New map mechanics.
Villagers are now more self aware.
Zombies and skeletons can now spawn with armor, and zombies now have a chance of turning a villager into a zombie villager, instead of the villager dying.
Zombie pigmen now have a chance to be able to pick up items.