Minecraft 1.8 is an upcoming update to the Minecraft client, it features new blocks, new commands, and even new skins.

New Splash Screen

New Difficulty Option

There is now an option which actually locks the difficulty of the game so that you cannot feel tempted to set the gamemode to peaceful when a creeper is about to explode. However, the /difficulty command will still work.

new commands

4th targeting system. (@e) which will target entities that can be specified to chickens, skeletons, etc.

/fill command which fills a given volume with a specified block

/clone command which clones all the blocks from one area to another

/blockdata command which modifies NBT data at a specific coordinate. The merges with the block at that position

/particle command which is used to create particle effects

NBT tags {candestroy} (for destroying items with a tool in adventure mode & {lock:"secret"} which is used to lock chests

new gamerule known as /logAdminCommands which is used to hide the command block's command from the chat

New Blocks & Items

Diorite (2 cobblestone & 2 nether quartz in a checker board position)
Andesite (1 cobblestone & 1 diorite)
Granite (1 nether quartz and 1 diorite)
(each of these can be polished with 4 of each in a 2X2 configuration)
Slime Block (used like a trampoline, but makes you move slower than walking on soul sand) (9 slime balls)

General Changes

Skins now have extra layers for the entire body, like what they did for hats, along with allowing the left & right arms, along with the left & right legs can be customized each

Villager trading is now based on occupation

Enchanting will not only cost you up to 3 EXP, but also up to 3 lapis lazuli, which spawns in larger veins

Changes

Doors are now stackable for up to 64, along with crafting it making 3 doors

when a furnace runs out of fuel, the smelting process pauses instead of resetting

Baby animals are added into the mix

When a villager is struck by lightning, it will turn into a witch