1.9 includes:
New mobs
Villager
Magma Cube
Snow Golem
Mooshroom
Blaze
New blocks
Netherrack Bricks
Netherrack Stairs
Netherrack Fence
Lily Pad
New items
Blaze Rod
Ghast Tear
Nether Wart
Gold Nugget
New generated structures
Nether Ruins
New Bugs
Biome tinting is too harsh between regular water and swamp water, rather than blending.
Can no longer jump repeatedly by holding a key
Snow golems do not take fall damage
Source
Also, from a Minecraft 1.9 wiki posting created prior to pre-release:
QUOTEFixes & Changes:
Will limit the amount of Experience Orbs dropped by a player on death.
Enderman block carrying will be nerfed.
New Monster Spawner texture.
New Features:
"Nether Update / Second Part of the Adventure Update".
This update will include new features in the Nether, such as:
Netherrack Bricks.
Netherrack Fence.
Netherrack Stairs
Nether Ruins.
Ghast Tear
Blaze Rod
Nether Wart
Gold Nugget
Two new nether mobs
Adventure mode
Finished Biome code and New Biomes.
Snowy Biomes.
Swamp Biomes will be adjusted to appear "swampier."
Mushroom Biomes.
Naturally generated Huge Mushrooms.
New Mushroom Cow, the Mooshroom.
Mycelium blocks.
New sound effects, 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.
6 New Mobs:
New Mob that is stronger than other Mobs (Possibly a Boss Mob).
Mooshroom Cows.
NPCs in Villages[16], 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 mobs for the Nether.
Both will be agressive.
Magma Cube
Blaze
Use for Experience (skills) will be added.
Animal breeding.
Increased number of Strongholds per map (from three to unlimited).
Boss Mobs in Strongholds.
New/Improved Armor system.
Increased armor durability.
Game mechanics changes
Blocking now moves the player's arm, rather than the sword just floating right in front of the player.
Players now have a firing animation for bows.