Too much to list. One minor thing I simply love is that if there's liquid one block above you, it drips through. I used this in The Nether at my portal room to make a bleeding ceiling. The lava sorta looks like blood this way, so I put lava over the top off the room. Too bad some morons tried to dig around the room and lava spilled out. In The Nether, it's loving impossible to stop unless you sit there forever placing bricks.
Then there's Mushrom Biomes (Islands within Ocean Biomes) which have Mooshrooms, red mushroom cows, that milk into bowls for Mushroom Stew and shear for 3-5 Red Mushrooms.
Snow Golems, which are snowmen with pumpkin heads. They wonder around and throw snowballs at aggressive mobs. You can put them in fence pens and put a lava pit under them. They throw snowballs (snowballs do no damage, but aggravate mobs) and the enemies walk into the lava.
The Nether now has Nether Ruins, which are structures built from Nether Brick, Nether Brick Stairs, and Nether Brick Fence. You can't craft Nether Brick yet, but you can craft it into the other two. They also have spawners for the new Blaze. They're golden looking things with puffs of smoke for bodies that fly around and shoot fire. Not to be confused with Ghast, the only things they have in common is flying and fire. Pigs drop Gold Nuggets, which can be made into Gold Ingots by putting 9 on a crafting table. Magma cubes roam, mainly in Nether Ruins.
NPC villages age Villager mobs, although all they do now is wander around.
http://www.minecraftwiki.net/wiki/Upcoming_features
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