Armor Specialization

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. This bonus 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,  Elemental and  Restoration Shaman and  Holy  Paladins.

Agility is considered the primary attribute for leather and mail wearing melee damage dealers and hunters, i.e. Feral  Druids in  cat form, all  Rogues, all  Hunters and  Enhancement Shaman.

Strength is considered the primary attribute for plate wearing melee damage dealers, i.e. Arms and  Fury  Warriors,  Frost and  Unholy  Death knights and  Retribution  Paladins.

Stamina is considered the primary attribute for tanks i.e. Feral  Druids in  bear form,  Blood  Death knights,  Protection  Warriors and  Protection  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.

Death knight
Plate Specialization (death knight): Increases your primary attribute by 5% while wearing Plate in all armor slots. Blood specialization grants Stamina, Frost specialization grants Strength, and Unholy specialization grants Strength.

Druid
Leather Specialization (druid): Increases your primary attribute by 5% while wearing Leather in all armor slots. Balance specialization grants Intellect, Feral specialization grants Agility while in Cat Form and Stamina while in Bear Form, and Restoration specialization grants Intellect.

Hunter
Mail Specialization (hunter): Increases your Agility by 5% while wearing Mail in all armor slots.

Mage
Wizardry: Increases your Intellect by 5%.

Paladin
Plate Specialization (paladin): 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.

Priest
Mysticism: Increases your Intellect by 5%

Rogue
Leather Specialization (rogue): Increases your Agility by 5% while wearing Leather in all armor slots.

Shaman
Mail Specialization (shaman): 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.

Warlock
Nethermancy: Increases your Intellect by 5%.

Warrior
Plate Specialization (warrior): 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.