Way back when Mists of Pandaria was announced, they talked about how there was going to be a zone in the center of the continent that you would do things in at the max level. This was, of course, the Vale of Eternal Blossoms, and while technically this description was true (though we went there at level 87,) the result was kind of disappointing. There were a bunch of daily quests, and dailies don't do a great job of telling a story - they're repetitive and inherently have to be a kind of gradual progress kind of campaign. There were one-shot quests at various reputation levels (a lot of this content got removed in 5.4) that did tell a bit of a story, but it was largely your standard "bad guy wants an artifact, you try to prevent him from getting it" thing.
Suramar, on the other hand, is unlike anything we've seen in WoW before.
First off, it's a real zone. It's not just a hub for daily quests. There's a plot and everything.
But it's also a plot that's very different from what we've seen before.
You'll get the quest to go to Suramar when you hit level 110. Almost every quest rewards artifact power, presumably so that there's something useful for anyone regardless of their gear level.
The story goes that Suramar was once the "jewel of the Night Elf empire." When the War of the Ancients came about, the mages in Suramar decided to create an utterly impenetrable arcane barrier to protect the city, regardless of what happened to the rest of Azeroth. Not only did this make communication impossible, but it also blocked out the sun and the moon - putting the entire city into an eternal night.
To survive, the elves used the powerful Nightwell - a font of magical energy that sounds a lot like the Sunwell or of course the Well of Eternity, though we don't know exactly what the origin of this one is yet. They lived off of pure arcane magic, sustaining their lives but also transforming them into a new subrace of elf - the "Shal'dorei" or "Nightborne."
For ten thousand years they got along all right. But as the Burning Legion invaded (this invasion,) they managed to send a message through the barrier - "give us the nightwell or die." The leadership of the city ultimately capitulated, and the people who rebelled were killed or exiled - and away from the power of the Nightwell, they began to wither into Nightfalen. Starved of mana, they would be doomed to eventually devolve into the "Withered."
It's a familiar story - the Blood Elves, having lost the Sunwell, turned to Fel magic to sustain themselves (this is why Blood Elf characters have green eyes as opposed to blue, which High Elves do.) But the Blood Elves did not literally live off of arcane magic for ten thousand years - Eversong Woods has plenty of normal fruit and such.
Anyway, you essentially have to help the Nightfalen organize a resistance movement, recruiting allies.
Early on, you gain the ability to don a Nightborne disguise, allowing you to enter the city of Suramar and avoid trouble as long as you're careful.
Suramar City is a vast urban area - it's certainly as large if not larger than the existing capital cities, and it really feels like a city that could support a significant population. It's a city where the guards are keeping people in line and collaborating with their demonic conquerors while the common people are too terrified to rebel because of what it would mean for them to be exiled.
There's intrigue, and there's great cloak-and-dagger moments - at one point a big elite Doomguard that could see through my disguise cornered me on a bridge and I snuck off onto a ledge over a canal to hide. Suramar is full of these moments.
On top of that, there's a huge number of quests. I seriously think there could be more quests in Suramar than the rest of the Broken Isles. There are of course World Quests as well, but a huge amount of content here is non-repeated, real story stuff.
And beyond the city there's tons of stuff as well. There's a vast underground ruin called Falonaar where there's a world quest every three days to lead a group of Withered (long story) and loot as much as you can while dealing with evil crazy spider-demon-elf things. These environments are seriously enormous.
And in 7.1 we'll be getting even more Suramar content. I'm totally on board for that, though I wouldn't mind seeing new campaign stuff added to the other zones as well. While we are getting another raid between the Emerald Nightmare and the Nighthold called the Trial of Valor, I still don't know what our "middle tier" is going to be (assuming that it's not Tomb of Sargeras and we don't know what the third tier will be.)