With the Alpha live (though still in a very early build in which only the Blizzcon version of the Demon Hunter starting experience is playable,) many an interesting detail has been discovered through quest text and voice files. This is a big expansion, with Warcraft's most important villains as the primary antagonists. Make no mistake, important things are happening, and the World of Warcraft will never be the same after Legion.
Spoilers Ahead, behind the Cut:
I've written earlier about the changing landscape of Alliance and Horde leadership. It looks like Varian Wrynn will die in the First Battle of the Broken Shore, and Vol'jin will go missing. Quest text indicates that Anduin is now King of Stromwind, though we have yet to find out if he becomes High King of the Alliance. Meanwhile, in Vol'jin's absence, Sylvanas Windrunner, as the senior racial leader within the Horde, is now Acting Warchief.
But that's not all. Varian appears to be only one of many longstanding characters who are doomed to die. We might be seeing George R. R. Martin levels of character dispatching.
First off, as many suspected, it appears that Tirion Fordring will be dying in the battle against the Legion. Retribution Paladins will find him mortally wounded and having lost the Ashbringer, possibly to Balnazzar (the dreadlord behind the Scarlet Crusade.) While the Paladin will retrieve the sword, it will be too late to save Tirion, who will be buried beneath Light's Hope.
Additionally, it appears that Ysera, the Green Dragon Aspect, will be corrupted by the Nightmare, and we will need to put her down ourselves. That leaves only Nozdormu and Alexstrasza left as the remaining original Aspects, and we already know (as he does) that Nozdormu will eventually fall to corruption (though I'd guess not in this expansion.)
Beyond character deaths, though, there are some other major developments.
Maiev, despite her recent out-of-game actions (including an attempt on Malfurion's life,) seems to be back to working with her brother Jerod in the fight against the Legion.
The Worgen will definitely be involved in an attack on the Forsaken - they sure as hell aren't happy that Sylvanas is there and taking power within the Horde.
We've gotten some new audio files as well that deal with the dragons and their loss of power after the defeat of Deathwing.
And finally, with audio that is presumably from the Frost Death Knight artifact quest, we learn that the Lich King (Bolvar, the... good?... one?) has given his blessing to the Knights of the Ebon Blade. He marks the Death Knight with his sigil, to be the Lich King's Champion and Enforcer of His Will. And then he sends you to hunt down Dreadlords and destroy them.