Ok, here's a break down of what is in the book... I'm actually pretty impressed at how much stuff is in there 0_o... the book is jam packed with stuff!
I also wonder at some of their call on 'elite' vs 'master' - granted, a Carnifex is pretty dumb, but good luck killing one without some kind of tank... it's certainly much mightier than a single battle brother! And some of those Master level opponent (looking at you Chaos) are downright insane.
Some of the stuff in this book is downright nasty as well... if you felt the game was too easy for your players, well, things are about to change. This may cause a problem for people who wanted to use this book for DH (non ascenscion game) or RT - you'll need to do a lot of scaling down (The Genestealer in this book eat the one from Creature Anathema for breakfeast as an example).
But here goes:
The Alien Threat:
Tau:
- A bunch of new weapon
- Broadside Battle Suit (Elite)
- Commander Flamewing (Master - a beefed up Commander)
- Ethreal (Master)
- Drone (Troop)
- Kroot Carnivor (Troop)
- Kroot Shaper (Master)
- Krootox (Elite)
- Knarloc (Elite)
- Great Knarloc (Elite)
- Kroot Hound (Troop)
- Tau Pathfinder (Troop)
- Vespid Stingwing (Troop)
Tyranids:
- 3 new weapons
- 7 new psychic power
- Carnifex (Elite)
- Dagon Overlord (Master)
- Gargoyle (Troop)
- Lictor (Elite)
- Purestrain Genestealer (Elite) & Broodlord (Master) - there is no rule on GS cults however
- Ravener (Elite)
- Ripper Swarm (Troop)
- Trygon (Master)
- Tyranid Warrior Prime (Master)
- Tyrant Guard (Elite)
- Venomthrope (Elite)
- Zoanthrope (Elite)
Orks:
- Ork Boyz (Troop)
- Meganob (Elite)
- Warboss (Master)
- Big Mek Wurrzog (Master)
Other Xenos (i.e., the guys who don't have army books in the TT):
- Bruul Parasite - Critter that infest humans and control them
- Crotalid (Troop) - a space crocodile basically
- Diablodon (Elite) - a space T-rex

- Lacrymole (Elite) - Shapeshifting humanoid aliens
- Locatl Mercenaries (elite) - quadrupedal lizard that are gun for hires.
Radical and Heretics:
- Inquisitor Thaddeus Hakk (Master) - some renegade inquisitor
- Apostate Cardinal (Master) - Basically a member of the clergy that turned to Chaos
- Pontifex Guard (Elite) - Their heavily armored bodyguard
- Magos Phayzarus (Master)
- Irradial Gogitator (Master) - Hal from space oddissey 2001 in 40k

- Samech Redemption Servitors (Elite)
- Slinnar War Machine (Elite) - Plasma powered cyborg
- Spire Slayer (Master) - animated doomsday device designed to wipe out Hive planets
The Force of Chaos:
- Bloodthirster (Master)
- Great Unclean One (Master)
- Keeper of Secrets (Master)
- Lord of Change (Master)
- Kyrus the Chantleader (Master) - an infamous chaos lord
- Bloodletter (Elite)
- Daemonette (Elite)
- Pink Horror (Elite)
- Plaguebearer (Elite)
- Chaos Space Marine Sorcerer (Elite)
- Khorne Bersekers (Elite)
- Noise Marines (Elite)
- Obliterators (Elite)
- Plague Marine (Elite)
- Possessed Chaos Space Marine Elite)
- Weapons for the Khornate, Noise and Plague Marines
- Chaos Psychic Power (5 total)
Advanced Adversaries:
- Advice on better using Hordes (psychic horde, hordes vers hordes, etc)
- About 17 new Horde traits
- Rule for Massed Battles
- Rules for turning points (Mechanic used in Final Sanction).
I'm personally thrilled with my purchase, as I got a lot more than I expected. 

And I'm somewhat the reverse of you. My favorite time, and the one I'm most familiar with, is the FedCom Civil War. Though that might have something to do with the fact I really wanted to see Katrina taken down.