I tried to cover all of the most important and exciting that was added for Minecraft 1.8. I know I did miss some items and blocks, but the entire list of blocks and items that have /will be added in Minecraft 1.8 are below:

Minecraft 1.8 Info:
Minecraft Wiki 1.8 Snapshots - goo.gl/sehD00
Minecraft 1.8 Overview List - goo.gl/Q41ksc

Minecraft 1.8 Overview:
Here’s a list of the major changes in the game:
▪ Added Granite, Andesite, and Diorite stone blocks, with smooth versions
▪ Added Slime Block
▪ Added Iron Trapdoor
▪ Added Prismarine and Sea Lantern blocks
▪ Added the Ocean Monument
▪ Added Red Sandstone
▪ Added Banners
▪ Added Armor Stands
▪ Added Coarse Dirt (dirt where grass won’t grow)
▪ Added Guardian mobs, with item drops
▪ Added Endermite mob
▪ Added Rabbits, with item drops
▪ Added Mutton and Cooked Mutton
▪ Villagers will harvest crops and plant new ones
▪ Mossy Cobblestone and Mossy Stone Bricks are now craftable
▪ Chiseled Stone Bricks are now craftable
▪ Doors and fences now come in all wood type variants
▪ Sponge block has regained its water-absorbing ability and becomes wet
▪ Worlds can now have a world barrier
▪ Added @e target selector for Command Blocks
▪ Added /blockdata command
▪ Added /clone command
▪ Added /execute command
▪ Added /fill command
▪ Added /particle command
▪ Added /testforblocks command
▪ Added /title command
▪ Added /trigger command
▪ Added /worldborder command
▪ Added /stats command
▪ Added a new player model with smaller arms, and a new player skin called Alex?
▪ Enchanting has been redone, now costs lapis lazuli in addition to enchantment levels
▪ Villager trading has been rebalanced