New achievements on the Midnight Beta suggests that dungeons in Midnight will have to be unlocked before players can start queueing for them.
There are eight new dungeons coming in World of Warcraft Midnight. In previous expansions, four dungeons would unlock over time, and hitting max level would unlock the remaining four. In Midnight, that is changing. Each dungeon does still have a level threshold, but can be unlocked earlier if you complete far enough into the campaign.
The eight achievements are:
Dungeon Unlock: Windrunner Spire 
Dungeon Unlock: Murder Row 
Dungeon Unlock: Den of Nalorakk
Dungeon Unlock: Maisara Caverns
Dungeon Unlock: The Blinding Vale
Dungeon Unlock: Nexus-Point Xenas
Dungeon Unlock: Voidscar Arena
Dungeon Unlock: Magisters’ Terrace
Windrunner Spire and Murder Row will unlock relatively early in your levelling, levels 81 and 83 respectively, but most others will be unlocked closer to max level or until max level, unless you complete certain quests. This will only be a thing for your first character, as these unlocks are account wide.
