That's right you are! It's what the Emperor would want!
Right now, actually, I'm getting ready to play it. But I'll post some info real quick.
*POSSIBLE SPOILERS (MINOR)*
It adds SO MUCH! Loads of (good) changes too. Let's see...
Adds a 5th race- The Imperial Guard
-Totally unique tech tree (nothing from other races)
-Defensive race (they have to be, they can win early game or late game, in between is hopeless)
Adds units to existing races
-Space Marine Chaplain (Imagine a Force Commander and a Chaplain combined. Yeah, he rocks. Cannot be attached to squads ;( )
-Orkz Mega Armored Nobz (Easily the best Ork infantry unit, and probablly one of the best units in the game)
-Chaos Khorne Berzerkers (The coolest unit in the game, period. They pretty much suicide charge the enemy, except they don't die in the process

)
-Eldar Fire Dragon (Never seen one used, supposed to be effective anti-vehicle and anti-building)