This guide is a simplified overview of all the changes in the Redstone Update (Java Edition 1.5.x releases), which assumes at least some familiarity with the Pretty Scary Update (Java Edition 1.4.x releases).
Mobs [ ]
Changed [ ]
Name
Image
Changed
All mobs
Mobs no longer walk over rails unless they are pursuing the player
Skeletons
Now shoot from 15 blocks away, instead of 10
Now shoot faster when their target is close, depending on difficulty
Slimes
Slime spawn rates in swamps now depend on the current moon phase
Zombies
Now group up on targets
They can now set players on fire when on fire themselves
Increased zombies attack damage
Increased player detection range
Blocks [ ]
New [ ]
Changed [ ]
Name
Image
Changed
Beacon
Increased effect radius from 16/24/32/40 blocks to 20/30/40/50 blocks
Brewing Stand , Chest , Detector Rail and Jukebox
Now emit a redstone signal into an adjacent comparator. Signal strength depends on the fullness of the block
Dispenser
Now equip armor to players that are up to 1 block away
Can place and activate TNT
Now use bone meal on a crop or sapling in front of them
Can create fire in front of them with flint and steel, which consumes durability
They can now face all 6 directions and use a different texture when facing up or down
No longer fire while updated, instead they fire when their input becomes powered
Now emits a redstone signal into an adjacent comparator. Signal strength depends on the fullness of the dispenser
Lava
Flows faster in the Nether
Updated texture
Nether Bricks
Items [ ]
New [ ]
Changed [ ]
Name
Image
Changed
Bone Meal
Now picks a random stage of growth each time, as opposed to automatically pushing them to the final growth stage
Saplings take 2–4 bone meal to sprout into a tree
Gives off green particles when used
Non-mob entities [ ]
New [ ]
Name
Image
Creation
Drops
Note
Minecart with hopper
Can only be emptied by passing over a hopper
Can pick up items from containers when it passes under them
Minecart with TNT
Gets triggered by activator rails
Explodes more violently the faster it goes
Doesn't destroy rails, supporting blocks or redstone materials
Minecart with Spawner
Can only be spawned in through commands.
Functions the same as a spawner, but can move like a minecart.
Structures [ ]
Changed [ ]
Enchantments [ ]
Changed [ ]
Name
Changed
Fortune
Now affects apples and potatoes.
Efficiency
Tools enchanted with efficiency now only speed up the mining of blocks that the tool is intended for.