With the Alpha live, though in a very limited form, some of the Order Halls for the various classes have been discovered. These will be areas that only members of the associated class will be allowed to enter, and while they have separate loading screens, it's not clear whether they will be truly instanced away or, more likely, use the garrison-style "seamless instance." Order Halls will not be yours alone - you'll have members of your class in there doing their thing as well.
So here's what we have so far:
Paladin: Light's Hope Chapel
It appears that the entrance to the Silver Hand order hall will be through a new stairway in Light's Hope Chapel, leading down to a vast vault.
The Shaman Hall is a large rock-structure overlooking the Maelstrom, with several kind of elemental hot-spots for the four elements. This looks like it will be largely open to the sky, but presumably only accessible to Shamans.
Rogue: Hall of Shadows, Dalaran
Rogues will have a special section in Dalaran's Underbelly. While I'm hoping the up the "opulence factor," for now it looks like your classic Thieves' Guild kind of location.
Mage: Central Tower in Dalaran
There appears to be a new tower in the center of Dalaran where the fountain/Icecrown Citadel memorial used to be (presumably this will be in the Broken Isles version of the city and not the Northrend one.) The Mage order will either be the Council of Tirisfal or a related offshoot, the Council of the Guardian.
Warlock: Legion Portal World
The Warlock area seems like a large collection of broken land floating somewhere in the Twisting Nether. I suspect their order will be the Circle of the Black Harvest.
Warrior: Halls of Valor
I don't quite know how this will connect to the dungeon, but Warriors will be able to hang out in the Warcraft equivalent of Valhalla, in Stormheim.
Hunter: Hunting Lodge, Highmountain
Hunters will have a nice outdoor area with a lodge in Highmountain, which seems to be a very classic "rugged wilderness" kind of location.
Priest: Unnamed Temple
We don't really have any indications of where the Priest hall will be, but it looks like it will branch off, with one dedicated Holy side and another dedicated Shadow side. Priests are flavorfully getting more evenly balanced between Light and Shadow, so this kind of makes sense.
Druid: Druidic Grove
Again, I don't know where this is - possibly Moonglade, possibly Val'sharah, but Druids will get a tranquil grove area for their class hall.
Monk: Monastery, Wandering Isle
Yes, Monks will be able to return (or go there for the first time if they aren't Pandaren) to the Wandering Isle, I suppose leaving the Peak of Serenity behind in favor of the big turtle.
Death Knight: Acherus, Broken Isles
Like Dalaran, Acherus has the advantage of being far more mobile than your typical town or fortress. It appears that Acherus, the Ebon Hold, will be floating above an archipelago in the Broken Isles and will continue to serve as the Death Knights' HQ (though I would assume you'll get a new, separate Death Gate to get to this version.)
Demon Hunter: Mardum
There is a big structure, possibly once the Titanic guard station on the prison world, though long, long ago corrupted by the Legion. After securing the Sargerite Keystone, it looks like Demon Hunters will return to the prison planet and claim it as their base.
So I believe that's it. It's very early in development, and we could see some significant changes by the time this goes live. I'm sure some of these halls are more fleshed out than others, so while I bet Light's Hope is rather polished at this point, places like the Rogues' Hall of Shadows are probably due for a lot of changes (I really hope there's an art gallery there with stolen pieces.)