If you started playing during Cataclysm or later, you might not realize how big expansion launch events used to be.
I started playing in September of '06, a few months before the launch of Burning Crusade. This was, obviously, their first expansion launch event. Doom Lord Kazzak, who used to roam the Blasted Lands, would come to the gates of various capital cities and destroy people. This was the first time I ever saw Core Hounds, and they were about twice the size of a dwarven tank. There was also a big event around the Dark Portal that I was sadly too low to participate in.
But Wrath went bigger. Wrath combined the Scourge Invasion - which was actually the second of such events, as the first had been part of the release of Naxxramas - with the Zombie Plague. Boxes appeared first in Booty Bay and then in capital cities that would infect players and NPCs alike with the zombie plague. After dying while afflicted (if I recall correctly,) players would turn into zombies and be able to infect others. They'd even get a new language called Zombie and were allowed to group cross-faction. Finally, the brand-new Stormwind Harbor and the less brand-new Valley of Strength would get invaded by Frostwyrms and Abominations that you could fight off. The Scourge Invasion involved several plague-spreaders sent around the world with undead who would drop necrotic runes - the runs could then be turned in at Light's Hope Chapel to get special gear, trinkets, and an Argent Dawn tabard (my most precious tabard.)
Cataclysm took things even farther. Before the pre-patch that re-did the old world, there were Elemental Invasions. Rifts would open around the world, allowing you to fight various elementals pouring out of them. Four cities were assaulted by different elements. Stormwind had Water Elementals attacking, so you'd have to lay sandbags to prevent flooding. Ironforge had Earth Elementals, and you'd have to reinforce the cavern with struts. Thunder Bluff was assaulted by Air Elementals, so you'd have to put up wind-guards along the edges of each bluff. Orgrimmar was assaulted by Fire Elementals, so you'd put up... riot barriers. After beating the invasions, temporary 5-player instances would open up, allowing you to fight level 80 versions of bosses from Maurodon, Blackrock Depths, Zul'Farrak, and a special new boss in AQ-20, all of which would drop ICC-quality epics.
Anyway, there was also an evolving quest chain where you'd infiltrate a doomsday cult that turns out to be Twilight's Hammer, and there were quests that established Thrall's transition from Warchief to Earth Shaman. You find the Titanic tablets that wind up turning Magni to diamond (well, if you're Alliance,) and you see Garrosh step up as Warchief.
Mists had the least fanfare - just a level 85 version of Theramore Falls. Warlords did have new stuff in the Blasted Lands and the first three bosses of the new UBRS (with Mythic dungeons, I've now done five different versions of that dungeon.) But they've both been pale comparisons to Wrath's and Cata's launch events.
So what do we have in store for Legion?
First off, there's the 40-person scenario on the Broken Shore, where we'll see the action immediately following the events of the Cinematic Trailer. The invasion clearly won't go well (many, myself included, suspect that Varian might die,) but presumably there will be rewards like a title, pet, and/or some gear.
The Legion will invade Azeroth, with Legion Citadels embedding themselves in the ground around Kalimdor and the Eastern Kingdoms, with events to clear out zones that are being affected.
Demon Hunters will become playable for those who have pre-ordered (and you can bet that I'll be pre-ordered by then.)
That's pretty amazing, especially having Demon Hunters playable before the launch. I'm really looking forward to this - if Blizzard can deliver everything they've been talking about, I seriously think it looks like a return to the WoW of old. Players who remember those days are older, of course, so it's always important to take one's nostalgia with a grain of salt, but I'm very happy with what I'm seeing about this.