New Achievements:

Amount of Experience Orbs dropped by a player on death now limited.
The ability of Endermen to carry blocks reduced in some way.
New Monster Spawner texture.
Ghast fireballs have a different texture.
On death, a proper score is given based on how many experience orbs the player has collected.
When flowing lava hits a water source directly from above, it creates Stone where previously the lava would flow through the water.
You can toggle between three states when pressing F5 (first person, third person back, third person front), having the addition of 3rd person view from the front.
Ender Pearls have a unique ability. When thrown, the player will teleport to the Ender Pearl's impact location, but take 2 and 1/2 hearts of fall damage. They will not work in Creative.
Golden Apples now glow purple in inventory.
Fences no longer block you from moving onto their space. You can now stand next to them, like you can with glass panes and iron bars.
They will also connect to solid blocks.
There is a slight delay between jumps when holding down the jump button, so that the player no longer bounces between ceiling and floor in two-block-high spaces.
Labels for items have a blue border.
Random loot chests in Strongholds.
Blocking now moves the player's arm, rather than the sword just floating right in front of the player.
Weapon and tool damage has been nerfed to make space for the increased damage enchantments (Diamond swords do 3.5 hearts now instead of 5).
Some mob health has been changed (Sheep, Spiders, Zombies)

New Features:

In the Nether:
Nether Bricks
Nether Fence
Nether Stairs
Nether fortresses
"The End" (unofficially called "The Ender", or "Enderlands"), which replaces the previously announced Sky Dimension
End Portals take the player from the Overworld to The End. To go back to the Overworld, one must defeat the Enderdragon and jump through the portal that appears.
End Portal Frame (Name not confirmed)
End Portal Block (Name not confirmed)
Endgame/Credit screen (You can still play your world)
End Stone

New Items:

Ghast Tear
Blaze Rod
Nether Wart
Gold Nugget
Glass Bottle
Spider Eye
Fermented Spider Eye
Blaze Powder
Magma Cream
Glistering Melon
Eye of Ender
Activates Ender Portal Frame blocks.
9 new Music Discs.
Finished Biome code and new Biomes.
Snowy Biomes.
Swamp Biomes adjusted to appear "swampier."
Darker Grass
Darker Water
Lily Pads
Mushroom Biomes.
Naturally generated Huge Mushrooms.
New Mushroom Cow, the Mooshroom.
Mycelium blocks.
New sound effects,[citation needed] including:
Minecart sounds.
Shears sounds.
Additional Bow sounds.
Big Slime sounds.
The true sounds of Endermen and Silverfish.
Changes in already existing sounds:[citation needed]
A change in the sound of Rain
New sounds for Doors
New sounds for the Skeletons' walk to make them sound more skeleton-y.
Bow models now appear in third-person larger and in 3D.
If water or lava is above a ceiling that is a single block thick, particles of water or lava will drip through, provided you are within 13 blocks of the ceiling.

New Monsters:
Enderdragon (Boss Mob)
Enderdragon eggs
Mooshroom Cows
NPCs in Villages, as Villager Mobs
Snow Golem mob.
Can be crafted
Melts in hot biomes
Spreads snow around when it walks
Passive, but throws snowballs at enemies
At least two new aggressive mobs for the Nether
Magma Cube
Blaze
Experience (skills) is now enchantments
Each Experience Orb is worth one Score Point
Animal Breeding
Wheat Bundles feed animals and make them come towards you
Feeding animals makes them enter "love mode"
Hearts will come out of animals while in love mode
Breeding will produce Baby Animals. Smaller versions of the parents with large heads that have a faster animation and a higher pitched sound.
Unlimited Strongholds per map (changed from three)
New/Improved Armor system
Increased armor durability
Sun and moon changes:
Moon phases
Sun and moon now rise in the east
Game mechanics changes:
Hardcore Mode - Locks the difficulty to Hard and forces you to delete your world if you die. The health bar changes appearance to indicate you are playing in Hardcore mode
Item Repair

Potions, Splash Potions, Potion effects, Brewing Stand, and Brewing
Enchantment Table, and Enchanting