What is Mastery?

Blizzard decided to introduce a new highly unusual stat in Cataclysm: Mastery. Mastery is a stat that has a different meaning for each of the 30 available World of Warcraft class specializations. In general, mastery is a stat that will massively increase the potency of a defining aspect of your chosen specialization and serve to significantly differentiate each spec.

You gain access to mastery via your class trainer; it becomes available during the period of leveling from 80 to 85. During this period a mastery rating will also begin to appear on gear. Your mastery is determined by your primary specialization, so if you're a Warlock that is speccing into Destruction your mastery will always be Fiery Apocalypse.

As soon as Mastery is implemented head to your trainer immediately to pick it up.

What does Mastery do for you?

Mastery will grant each spec of each class a unique scaling buff. It's wildly different for most classes, though there are some similarities like the masteries for Marksmanship, Combat, and Arms as well as those of the two Restoration trees. Here is the complete collection of all the new Mastery abilities:

Death Knight

Blood - Blood Shield: Each time you heal yourself via Death Strike, you gain 50% of the amount healed as  a damage absorption shield. Each point of Mastery increases the shield by an additional 6.25%.

Frost - Frozen Heart: Increases all frost damage done by 16%. Each point of Mastery increases frost  damage by an additional 2.0%.

Unholy - Blightcaller: Increases the damage done by your diseases by 32%. Each point of Mastery  increases disease damage by an additional 4.0%.

Warrior

Arms - Strikes of Opportunity: Grants a 16% chance for your melee attacks to instantly trigger an  additional melee attack for 75% normal damage. Each point of Mastery increases this chance by  2%.

Fury - Unshackled Fury: Increases the benefit of abilities that cause or require you to be enraged by  25%. Each point of Mastery increases enrage effects by an additional 3.13%.

Protection - Critical Block: Increases your chance to block by 10% and your chance to critically block by  10%. Each point of Mastery increases your block and critical block chance by an additional  1.25%.

Rogue

Assassination -Potent Poisons: Increases the damage done by your poisons by 20%. Each point of   Mastery increases damage by an additional 2.5%.

Combat - Main Gauche: Your main hand attacks have a 10% chance to grant you an extra off hand  attack. Each point of Mastery increases the chance by an additional 1.25%.

Subtlety - Executioner: Increases the damage done by all your finishing moves by 20%. Each point of  mastery increases damage by an additional 2.5%.

Hunter

Beast Mastery - Master of Beasts: Increases the damage done by your pets by 32%. Each point of  Mastery increases pet damage by an additional 2.5%.

Marksmanship - Wild Quiver: Grants a 26% chance for your ranged attacks to also instantly fire an  additional ranged shot for 50% normal damage. Each point of Mastery increases the chance by  an additional 2%.

Survival - Essence of the Viper: Increases all elemental damage you deal by 32%. Each point of Mastery  increases pet damage by an additional 2.5%.

Druid

Balance - Total Eclipse: Increases the bonus damage from Eclipse by 12%. Each point of Mastery  increases the bonus by an additional 1.5%.

Feral - Savage Defender (Bear): Increases the damage absorbed by your Savage Defense ability by 32%.  Each point of Mastery increases the absorb by an additional 4%.
Razor Claws (Cat): Increases the damage done by your bleed abilities by 20%. Each point of  Mastery increases bleed damage by an additional 2.5%.

Restoration - Symbiosis: Increases the potency of your heal over time spells by 10% on targets already affected by one of your heal over time spells. Each point of mastery increases heal potency by an additional 1.25%.

The "Symbiosis" Mastery is activated immediately upon training Mastery on this Restoration Druid.

Mage

Arcane - Mana Adept: Increases all spell damage done by up to 12%, based on the amount of mana the  Mage has unspent. Each point of Mastery increases damage done by up to an additional 1.5%.

Fire - Flashburn: Increases damage done by all your periodic fire damage effects by 20%. Each point of  Mastery increases periodic damage done by an additional 2.5%.

Frost -  Frostburn: All your spells deal 20% increased damage against Frozen targets. Each point of  Mastery increases damage by an additional 2.5%.

Warlock

Affliction - Potent Afflictions: Increases all periodic shadow damage you deal by 22%. Each point of  Mastery increases periodic shadow damage by an additional 1.63%.

Demonology - Master Demonologist: Increases the damage done by your demon servants and damage  you deal while transformed into a demon by 20%. Each point of Mastery increases damage by  an additional 1.5%.

Destruction - Fiery Apocalypse: Increases all fire damage you deal by 17%. Each point of Mastery  increases fire damage by an additional 1.25%.

Shaman

Elemental - Elemental Overload: Grants a 29% chance for Elemental Overload to occur. Elemental  Overload causes a Lightning Bolt, Chain Lightning, or Lava Burst spell you cast to trigger a  second, similar spell on the same target at no additional cost that causes 75% of normal damage  and no threat. Each point of Mastery increases the chance of Elemental Overload by an  additional 2%.

Enhancement - Enhanced Elements: Increases all Fire, Frost, and Nature damage done by 36%. Each  point of Mastery increases damage by an additional 2.5%.

Restoration - Deep Healing: Increases the potency of your direct healing spells by up to 36%, based on  the current health level of your target (lower health targets are healed for more). Each point of  Mastery increases direct heals by up to an additional 2.5%.

Paladin

Holy -  Illuminated Healing: Your healing spells also place an absorb shield on your target for 8% of the  amount healed lasting 6 sec. Each point of Mastery increases the absorb amount by an  additional 1%.

Protection - Divine Bulwark: Increases your chance to block melee attacks by 16%. Each point of Mastery  increases block chance by an additional 2%.

Retribution - Hand of Light: Your autoattacks have an 8% chance to grant Hand of Light, causing your  next Holy Power ability to consume no Holy Power and to cast as if 3 Holy Power were  consumed. Each point of Mastery increases the chance by an additional 1%.

Priest

Discipline - Shield Discipline: Increases the potency of all your damage absorption spells by 20%. Each  point of Mastery increases the potency of absorbs by an additional 2.5%.

Holy -  Echo of Light: Your direct healing spells heal for an additional 10% over 6 sec. Each point of  Mastery provides an additional 1.25% healing over 6 sec.

Shadow - Shadow Orbs: You have a 10% chance for your Shadow Word: Pain and Mind Flay spells to  grant you a Shadow Orb each time they deal damage. The Shadow Orb increases the damage  done by your Mind Blast and Mind Spike spells by 20%. Each point of Mastery increases damage  by an additional 2.5%. You can have up to a maximum of 3 orbs.

Mastery Scaling

As many will have already noted, the mastery ratings on gear, even green pieces, can be upwards of 100. You might be wondering how the hell they will possibly balance the game if 'each point of mastery' can give you 2% damage or higher. Those descriptions are a little bit deceptive.

Mastery is a rating much like critical strike, haste, hit, or expertise. Each character starts off with a base mastery of 8 points and each additional point of mastery requires 180 mastery rating at level 85. Mastery is a linearly scaling stat which means that if you have 180 mastery rating you'll have 9 mastery  (8 base +1 from rating) and if you have 1800 mastery rating you'll have 18 mastery (8 base + 10 from rating).

Mastery appears like any other combat rating but its exact value is far more difficult to determine.

Due to the fact that mastery can trigger procs, add flat damage, or add damage to very specific aspects of your character means that the way it will interact with your other stats will be extremely complicated. Mastery will take theorycrafting and min/maxing to an entirely new plane, which is ironic because one of Blizzard's stated goals for Cataclysm and the rationale for removing stats like Armor Penetration was to simplify character stats. Mastery will inevitably be better for some classes than others and may lead to some extremely strange or cheesy builds that involve stacking mastery or avoiding it completely.