One of the major features of the pre-launch event for Legion is the Battle of the Broken Shore. This is a scenario that allows up to 80 players (40 Alliance and 40 Horde) to fight through the Broken Shore - the area of the Broken Isles with the Tomb of Sargeras, which will almost certainly be the final raid of the expansion (unless the third tier is a curveball, in which case it might be the second.)
Big things happen here, so I'm going to put this all under a spoiler tag. There are quests that immediately follow it, but they end kind of abruptly, so I wonder if there's more to come.
Anyway, spoiler break:
Before you head off, you can get some very powerful buffs to help you survive the battle. I ran this as a Protection Paladin and a Subtlety Rogue, and both worked out fine, though the Rogue came pretty close to dying once (no Cheat Death though.) You'll be escorted by some powerful allies - major racial leaders as well as a big army of NPCs, though I suspect that the big army might actually be placeholders for other players, protecting you if you queue when there aren't a lot of people queueing up for it.
Both sides arrive in ships that crash into the shore after a cutscene that has not yet been implemented. Alliance players come to back up Jaina and Genn, Horde meet up with Vol'jin and Thrall. After destroying some towers, you face off against a boss-like adversary and then move forward.
Alliance meets up with Mekkatorque (in a badass mech) and Varian, Horde meets up with Baine and Sylvanas. You fight your way through a massive Legion camp until enough of it has been eliminated to move on. The Legion retreats from this area, drawing you upward to where Tirion Fordring has been captured by Gul'dan.
Gul'dan summons Krosus (who I believe is a boss in the Nighthold,) who burns through Tirion's Divine Shield and then dumps him into a bunch of Fel lava (anyone who's not a Paladin might assume this is where he dies, though the Ret artifact weapon quest has you attempt to rescue him from I believe Balnazzar.)
Alliance and Horde alternate between fighting Krosus and a bunch of adds that spawn while the other faction is busy with him. The Alliance gets him first.
After this, the Alliance heads further up to confront Gul'dan, only to encounter a massive Legion force that includes many demons we thought we had slain, like Jaraxxus, Brutallus, and Tichondrius. After fighting a few waves of these guys, the screen blacks out and we get another missing cutscene.
Horde forces go to attempt to cover the Alliance from above, but they discover a massive set of portals and their own endless onslaught of demons. Again, after a few waves, the screen blacks out and there's another missing cutscene.
Alliance forces awake aboard an airship. Apparently within this cutscene, Varian was killed, and Genn blames the Horde for abandoning their forces to the Legion (I suspect something else was going on.) He sends you to Stormwind Keep while he sets off to get vengeance, unable to face Anduin with such a loss.
You can take a griffin, escorted by Tess Greymane (Genn's daughter) and Lorna Crowley. You find Stormwind Keep in mourning, and you present Anduin with a letter his father was writing to him (presumably the one he's writing in the cinematic.) Anduin, now king, accepts the condolences of the Alliance leaders who are present while Jaina argues that they have no obligation to protect the Horde, especially after... whatever happened in the cutscene. After this conversation, a member of the Illidari appears, telling you that there are demons lying in wait to assassinate the Alliance leadership. He exposes them and you fight through the demons and close their portals. Afterward, Anduin sends you to serve as ambassador to the Illidari, who have been granted a spot in the Mage District.
Horde forces leave the cutscene at the docks, where Eitrigg tells you that the Warchief wants you to attend a secret meeting in Grommash Hold. You arrive in the Valley of Strength and Saurfang lets you into the fortress.
Vol'jin is on the verge of death, stabbed by a cursed blade whose poison is eating away at him. He wishes to name his successor soon, especially amidst rumors that the Alliance wants revenge. There's another cutscene that is missing, and then you're outside of Orgrimmar where there's a huge gathering greeting Sylvanas as the new Warchief (flanked by Nathanos Blightcaller, who has a new model, more of a human with Death Knight skin-rot and red eyes,) standing in front of Vol'jin's funeral pyre. You swear loyalty to her just as an Illidari representative comes by and warns you that the Legion has infiltrated this ceremony. You fight through the demons and Sylvanas tells you to serve as her ambassador to the Illidari (and also to spy on them, which as an Undead Rogue made my heart skip a beat - or it would if it were still beating.)
So there you have it - some really big shake-ups in the leadership of both sides. Anduin is basically all you could hope for in a bright young King, though the current trials are likely to test him. The Horde, meanwhile, has got to be super nervous - even if they successfully fend off the Legion (which is probably a safe bet,) Sylvanas has never been the most ethically-bound leader, and the memory of Garrosh is, I would bet, pretty fresh in the Horde's collective minds. Honestly, I'm hoping that Sylvanas takes her ascension as a reason to really reform her ways - let Genn be the bad guy in this rivalry for a little bit.
Point is: both factions are in a bit of disarray right as the Legion's invasion is beginning. This is a really major fight, and Azeroth is going to have to play catch-up if they're going to win.