The replacement for garrisons, and hopefully a vast improvement, these are large zones unique to each class. Everybody gets one, be it the Warrior Halls of Valor straight from the Vrykul afterlife, or the Priest Netherlight Temple - a spaceship housing a Na'ru.
This is where you upgrade your artifact, start class-specific quests and where you enter new zones. Everyone has a special teleport skill to return to theirs, so getting back and forth should be easy. You'll become the master of your class hall (everybody's the king) at the end of your artifact quest, hence why you get to boss people about.
Order Hall's will also have Garrison-style missions to send people on, but these are now split into Champions - big important people from the lore, who there will be very few of - and Troops - expendable soldiers you'll buy for resources.
Here's all of the Class Halls: