MinecrafWiki...the ultimate source for everything about minecraft...you should really check it out.

Especially the upcoming fearures page...which is everything about 1.8 for right now.

Here's some new stuff:

Enderman will take damage from water and emit black smoke particles.*So I guess you can use a bucket of water for defense :D*

A new blue spider mob that will spawn in abandoned mineshafts and will have a unique mob spawner and have venomous attacks..

Shifting while hitting a mob will send them flying around 8 blocks.

New achievments: Kill a skeleton with an arrow at 50 meters.

Possibly attempting to shoot the moon with an arrow.

in Creative Mode you will be able to change time quickly and easily...and will have unlimited blocks.

A new menu *I think*

Silverfish will be rare.

Optional Feature *turn on/off I guess?* NPC vilages.they will have "town borders" and farms and wells.

Ravines both underground and on the surface with ledges to walk on.

Ruins/Strongholds.

Fence Gates.

Atleast 3 *THREE!!!* New mobs.

Modes:Adventure Mode,The regular mode we have now *I wrote this so noone will get confused with adventure mode*,Creative Mode,and Realistic mode *I guess*

New eating motions.

Oh and in Minecraft 1.9 there is supposed to be a "Nether Update". *Nether update is in 1.9 not 1.8*

Well thats it for now.Thats NOT ALL thats going to be added.

Well I cant wait any longer!!!!!

You should really check out the upcoming features on MinecraftWiki.