This guide is a simplified overview of all the changes in the Java Edition 1.3.x releases, which assumes at least some familiarity with the Java Edition 1.2.x releases.
Mobs [ ]
Changed [ ]
Name
Image
Changed
All mobs
Mobs can now spawn on top of all upside down slabs and stairs
Pigs
Drop 1–3 porkchops instead of 0–2.
Silverfish
Will now spawn more silverfish from nearby silverfish blocks more often, when suffering from poison potions.
Creepers
Now have increased knockback effect.
Now do more or less damage depending on difficulty:
Easy: ~50% less damage
Normal: same damage as before
Hard: 33 1⁄3% more damage.
Endermen
Now stop in front of the player.
Now teleports behind the player when the player swings their sword.
Villagers
Will now get a random profession when spawned from spawn eggs.
Blocks [ ]
New [ ]
Changed [ ]
Name
Image
Changed
Wooden slabs , and Wooden stairs
Added all wood types.
Wooden slabs crafted after the update act like wood.
Adjusted hitbox.
Wooden pressure plates
Can be triggered by arrows.
Log
Can now be placed in 3 different directions.
Placement works similar to piston placement.
Furnaces
Can now be fueled by wooden tools.
Now return an empty bucket when using lava buckets as fuel.
Ice
Now disappears instead of melting when placed in the Nether, regardless of light.[verify ]
Can now be obtained using Silk Touch-enchanted tools.
Glass panes
Can now be obtained using Silk Touch-enchanted tools.
Dispenser
Minecarts & boats will now be placed if there's rails/water in place respectively.
Will now suck in or place water or lava in front of them instead of dispensing buckets.
Gravel
Levers
Can be placed on the underside of blocks.
Pistons
Now take 2 redstone ticks (4 game ticks) to extend.
TNT
Now does different amounts of damage depending on difficulty:
Peaceful: no damage
Easy: ½ less damage
Normal: same damage as before
Hard: ⅓ more damage.
Nether wart
Can now be grown in the Overworld and in the End.
Signs
Now stack up to 16.
Crafting now gives 3 signs instead of 1.
Monster spawners
Added NBT tags to further customize spawned mobs.
Only changeable using map editors or mods.
Doors
Now show their damage when being broken by zombies.
All Slabs & stairs
Will now be placed upside-down when placed against the upper half of a block's side.
When placed upside down, rails, pressure plates, levers, doors, torches, redstone torches, repeaters, redstone dust and beds can be placed on them - Redstone dust behaves like on glowstone - It transmits power horizontally and upwards, but not downwards. Redstone torches don't power redstone placed on slabs/stairs above them.
Items [ ]
New [ ]
Name
Image
Obtaining
Uses
Emerald
Is used to trade with villagers
Book and quill
Right-click to read or write in and edit up to 50 pages.
Clicking 'Done' saves the text edited into the book
Clicking 'Sign' chooses a title to finalize the book
Structures [ ]
New [ ]
Name
Image
Generates
Chests
Mobs
Description
Desert village
Generates in deserts
Can generate blacksmith buildings with 1 chest.
Villagers
Desert village variant made from sandstone instead of cobblestone and wood.
Desert pyramid
Generates in deserts
4 chests
None
Built out of various kinds of sandstone and wool.
Includes hidden chest room and TNT trap, with loot including rotten flesh, bones, iron ingots and gold ingots, diamonds and emeralds.
Jungle pyramid
Generates in jungles
2 chests
None
Built out of various kinds of cobblestone.
Contains multiple tripwire traps triggering dispensers filled with arrows.
Loot chests include bones, rotten flesh, gold ingots, iron ingots, diamonds and emeralds.
World types [ ]
New [ ]
Name
Image
Description
Structures
Large Biomes
Generate as default worlds, but 16× bigger
Beaches look closely to what they looked like in Java Edition Beta 1.7.3 and prior.
Same, as in normal world.