The armor specialization bonus is a 5% increase of your primary attribute while wearing your correct armor class (plate, mail, leather or cloth) in all slots. It was added in Patch 4.0.1 in an effort to discourage players from 'downgrading' and to encourage them to wear armor of the highest armor class possible.
For hybrid classes (excluding priests) a character's specialization will determine the attribute increased by the armor specialization bonus. Pure classes and priests get the same bonus regardless of their specialization.
Intellect is considered the primary attribute for healers and offensive casters, i.e. all mages, all warlocks, all priests, Balance and Restoration druids, Mistwalker monks, Elemental and Restoration shaman and Holy paladins.
Note that the bonus gained by the pure classes (hunters, mages, rogues and warlocks) is independent of their specialization. This also applies to priests who are a hybrid class because both their roles (healing and magical DPS) benefit from the same attribute (Intellect).
Also note that the bonus for the cloth wearing classes (mages, priests and warlocks) does not have any armor requirement. This is because these classes can only wear cloth armor and cannot 'downgrade' to a lower armor class.
Armor Specializations by class
: Increases your primary attribute by 5% while wearing only Leather armor. Balance specialization grants Intellect, Feral specialization grants Agility, Guardian grants Stamina while in Bear Form, and Restoration specialization grants Intellect.
: Increases your primary attribute by 5% while wearing Leather in all armor slots. Brewmaster specialization grants Stamina, Mistweaver specialization grants Intellect, and Windwalker specialization grants Agility.
: Increases your primary attribute by 5% while wearing Plate in all armor slots. Holy specialization grants Intellect, Protection specialization grants Stamina, and Retribution specialization grants Strength.
: Increases your primary attribute by 5% while wearing Mail in all armor slots. Elemental specialization grants Intellect, Enhancement specialization grants Agility, and Restoration specialization grants Intellect.
: Increases your primary attribute by 5% while wearing Plate in all armor slots. Arms specialization grants Strength, Fury specialization grants Strength, and Protection specialization grants Stamina.