30% Armor Buff In-world, Mage Tower, Dungeons, All Classes (Patreon)

The starting scenario for Legion has a couple of preliminary quests in Stormwind from Recruiter Lee in the harbour, the second of which is "To Be Prepared". This gives you a "Polished Armor" buff which increases your armour by 30% and lasts for an hour. 

The buff is amplified by any armour enhancing passive abilities your character has. For example my 110 priest normally has 1661 armour and 18.35% physical damage reduction, with the buff 2159 armourand 22.61% physical damage reduction.

The buff is active in world and dungeons. It does not seem to be active in instanced pvp or mythic dungeons. This is invaluable for levelling and world questing at lower gear levels.. 

You do not have to complete this quest to get to Dalaran, simply start the quesr and skip to Dalaran directly by talking to Recruiter Lee.  You can re-access the buff whenever you like by returning to the harbour.

As the pop-up text mentions you can re-access the quest if you drop it via Archivist Elysiana in Dalaran. The text doesn't actually mention where she is-she's on the left of the Violet Citadel as you enter.

(Disclaimer: Originally many of the "To be prepared" buffs were exploitable at the start of Legion which several people pointed out at ownedcore.com. Most were fixed. I have not seen any one mention any of them working since through two major patches. For this reason I consider this effectively  new, useful information rather than a rehash.)

EDIT: This works in the Mage Tower challenges also.