Now that WoW Chronicle Vol. 1 is in the hands of reviewers, we're getting some pretty massive spoilers. There's one kind of meta-spoiler (as in it relates to a lot of other things) that I'm going to focus on here. So here's a buffer for people who might be on the mobile site or something. These are massive, lore-underpinnings-spoiling spoilers that you should only look at if you really want to know and don't want to get the book (or wait to get the book.)
It turns out the Big Bad of the Warcraft Universe is neither Sargeras nor the Old Gods. It's the Void, and the Void Lords who inhabit it. Theoretically, the Void Lords have never been able to manifest in physical reality themselves. What that means for Voidwalkers and their ilk, I'm not sure, but the Void Lords created something in order to spread the Void into physical reality. Their tools? The Old Gods.
The Void Lords were the ones who the Nathrezim were serving when Sargeras encountered them. Through them, he learned of the existence of the Void and the Old Gods, which the Void Lords would use to attempt to corrupt the nascent Titan World-Souls growing throughout the universe.
Knowing that the World Souls were vulnerable to corruption, Sargeras set about destroying them, rather than allowing them to be corrupted, as well as any other planets that could harbor this threat (meaning all of them.) When he killed his first World Soul, Aggramar attempted to stop him and reason with him. They fought and Aggramar fled, informing the Pantheon of what he was doing. They agreed to meet on a planet called Nihilan, and Aggramar approached Sargeras unarmed, inadvertently revealing the existence of a new Titan named Azeroth in an attempt to persuade Sargeras to set aside his Burning Crusade. Instead, Sargeras killed Aggramar and a war between the Titans and the Burning Legion began.
And the Titans lost.
Unable to counter his use of Fel magic, the Titans were slain by Sargeras. Golganneth tried to use a spell to preserve their souls as their bodies fell, intending to inhabit the bodies of the Keepers on Azeroth, but the spell only allowed them to give the Keepers brief visions before the Titans faded from existence.
Just as Sargeras had feared, the Void Lords had sent Old Gods to Azeroth to try to corrupt it. The Titans had had the most powerful of them, Y'shaarj, destroyed, but this tore open a wound in the nascent Titan. Its blood pooled into a font of incredible magic energy which would come to be called the Well of Eternity.
Fearing that anything in the Great Dark Beyond might be corrupted by the Void, Sargeras has been attempting to destroy any potential hosts.
So that's the biggest thing.
Now some additional tidbits:
Odyn was the original prime designate. He objected to the idea of empowering the Dragon Aspects, given how they had fallen to the Curse of Flesh, and decided to imbue the Titans' greatest warrior race, the Vrykul, with that power instead. He separated off a part of Ulduar to serve as his Halls of Valor, where he would allow the greatest warriors to escape death and instead live with him forever (shiny and chrome!)
All creatures that die on Azeroth go to the Shadowlands (I JUST REALIZED THAT'S WHERE YOU GO WHEN YOU'RE A GHOST IN-GAME!) and so he made some select vrykul women into Val'kyr to fetch worthy souls from the Shadowlands and take them to the Halls of Valor. The vrykul were not keen on this, and so he forced one named Helya to become the first. She rebelled and created a different death-realm called Helheim. Some Val'kyr rebelled against both sides, bringing what souls they could find in the Shadowlands back to life - these Val'kyr are Spirit Healers.
Odyn's a jerk. Why aren't we killing him?
Algalon is from a race that was actually not created by the Titans, and is called the Constellar. Still, they like the Titans and joined up with them.
Mimiron looks like a giant mechagnome because Loken killed his original form and the mechagnomes built him a new body. But they flubbed it a bit, which is why he became a mad inventor and wasn't helping to stop Loken.
Vrykul, Earthen, and Mechagnomes went south with Tyr and Archaedas. The Earthen were so afraid of the Curse of Flesh that they asked Archaedas to seal them away in Uldaman. The Mechagnomes offered to stay and stand guard, even if it meant that they would succumb.
The original Vrykul of Tirisfal are not the ancestors of the humans. When the humans and their Vrykul parents arrived, the earlier settlers had gone elsewhere (possibly Stormheim?)
On ancient Kalimdor, there were just the four Old Gods we know of.
BUT there were other continents than Kalimdor before the Sundering.
The Furbolgs are descended from a more feral bear race who fought the Vrykul.
The trolls awakened a C'thraaxi (General Vezax's race) thinking it was a new loa, which set off the Troll/Aqir war. The Amani eventually killed him and built Zul'aman over his corpse.
Sargeras looked like a normal Titan until he destroyed Mardum. The Fel energy released disfigured him.
That's what I've got so far. Definitely paints a darker picture of the Warcraft Universe. Sargeras seems to be kind of still doing his job, just doing so in such a way that it invalidates his entire purpose.
Anyway, I told you they'd be big spoilers, didn't I?
EDIT: More Stuff!
The Yaungol were the original Taurenoid race. They were followers of Cenarius who went south and were enslaved by the Mogu Emperor Qiang the Merciless. The Mogu transformed them to serve as better slaves. When the Pandaren overthrew the Mogu, some Yaungol stayed around to fight the Mantid while others went north, eventually hitting the Well of Eternity and getting transformed into the forms we know today.