Minecraft wiki entry for Adventure_Update, associated text: A proper ending to the game in a realm called "The End".
New Nether content.
Nether bricks (called nether brick at the time)
Nether brick stairs
Nether brick fence
New achievements.
Score is stated on death based on total experience orbs held on the player.[5]
A new game mode. (Hardcore)
New biomes. (Mushroom biome, tundra reintroduced)
New items.
Eye of ender
Blaze powder
Blaze rod
Glistering melon
Glass bottle
New texture for some blocks.
Block of iron
Block of gold
Block of diamond
Dirt
Grass block
Stone
Planks
Gravel
Netherrack
Glowstone
Slab-Bug fixed.
New blocks.
End stone
Brewing stand
Cauldron
Enchantment table
End portal frame
New mobs. (Ender dragon (Boss Mob),[6][7] snow golem,[8][9] mooshrooms,[10][11] villager mobs,[12] magma cube, blaze)
Animal breeding.[13][14]
Potions.
Sword rebalance as well as enchantments.
Improved Armor system, with slightly more durability.[15][16]
Mob health rebalance. (Sheep, spiders, zombies)
New sound effects, including: chest sound, door/gate sound, bow Sound, enderman sound, silverfish sound, explosion sound, egg sound, eating sound, sound when XP orbs are collected, sound of bones cracking after fall damage is taken, sound of a broken tool.
Item Repair.
New stronghold content (Portal rooms with the end portal frames).
Bow now has durability and infinite arrows in creative.
Lily Pads that can be placed on water and stood upon.
Many blocks are mined faster:
Axes mine fences, wooden doors, wooden pressure plates and crafting tables faster.
Pickaxes mine obsidian and redstone ore faster.
Redstone placed on its own no longer has a "plus" format, instead it has a small clump.
Added a World Type option. (Not used until Java Edition 1.1 introduced Superflat)