There are three core features:
The 5-player Karazhan dungeon is obviously the headlining feature. From the video preview, it's clear that this will be a mix of new and old - there are familiar environments but also totally new ones. Boss-wise, we saw Maiden of Virtue, the Opera (though with new plays this time,) and Medivh (who I'd guess is the final boss, but we'll see.)
While it's probably going to be familiar at first, once you get a bit farther in things go totally insane - you'll find some kind of vortex that teleports you to the upside-down Karazhan - yes, that's confirmed finally. There you'll fight your way around the upside-down world and into what might be the Twisting Nether or some other weird arcane space. There seems to be a chessboard fight (though one that you actually fight in) on a board that is breaking apart in this weird space.
Karazhan is tuned to be harder than Mythic dungeons, so they're going to be generous with the gear quality.
As a reminder - this will not be replacing the old raid - it's a new instance with its own entry, though lore-wise some of the same locations will feature.
The other big instance is the Trial of Valor, which is a 3-boss raid in Stormheim. You begin in the Halls of Valor, fighting Odyn again (to knock him down a bit more) and then when you have proven yourself to him, he sends you into Helheim. You fight Guar, the three-headed dog you see while questing, and then fight Helya again (though this time presumably to the death.) Apparently there will be some lore stuff about Illidan here. This is, I believe, meant to come out before Nighthold but after Emerald Nightmare (and I'd assume it'll be a single Raid Finder wing.)
Finally, new weekly campaign quests will be added to Suramar to continue the story of the Nightfalen's resistance against Elisande. Given the huge amount of quest content in Suramar (seriously, I think there are more quests there than the rest of the Broken Isles zones combined) it's clear that they're really taking this story seriously. I've been enjoying the quests, so no complaints here.
Well, five days in Legion is pretty amazing. And if they can keep up content like this at a good pace I think we're all in for a fantastic expansion.