http://wow.joystiq.com/2011/05/05/patch-4-2-ptr-tier-12-set-bonuses/ said:
Death knight DPS:
2-Piece - Your Horn of Winter ability also grants you 3 runic power every 5 sec for 2 min.
4-Piece - Your critical strikes with melee abilities deal 15% additional damage as Fire damage over 4 sec.
Death knight tanking:
2-Piece - Your melee attacks cause Burning Blood on your target, which deals 800 Fire damage every 2 for 6 sec and causes your abilities to behave as if you had 2 diseases present on the target.
4-Piece - Your Dancing Rune Weapon grants 15% additional parry chance.
Druid (balance)
2-Piece - You have a chance to summon a Burning Treant to assist you in battle for 15 sec when you cast Wrath or Starfire.
4-Piece - While not in an Eclipse state, your Wrath generates 3 additional Lunar Energy and your Starfire generates 5 additional Solar Energy.
Druid (feral)
2-Piece - Your attacks with Mangle, Maul, and Shred deal 10% additional damage as Fire damage over 4 sec.
4-Piece - Your finishing moves have a 20% chance per combo point to extend the duration of Berserk by 2 + (20 * ComboPoints) sec and your Barkskin ability grants an additional 10% chance to dodge for 12 sec.
Druid (restoration)
2-Piece - Your periodic healing from Lifebloom has a 40% chance to restore 1% of your base mana each time it heals a target.
4-Piece - When your Lifebloom blooms, it instantly heals up to 2 nearby injured targets for the same amount.
Hunter
2-Piece - Your Steady Shot and Cobra Shot have a 10% chance to trigger a Flaming Arrow, dealing 0% instant weapon damage as Fire.
4-Piece - You have a 10% chance from your autoshots to make your next shot cost no focus.
Mage
2-Piece - You have a chance to summon a Mirror Image to assist you in battle for 15 sec when you cast Frostbolt, Fireball, or Arcane Blast.
4-Piece - Your spells have an increased chance to trigger Brain Freeze, Hot Streak, or Arcane Missiles. In addition, Arcane Missiles is now castable while moving.
Paladin (holy)
2-Piece - Casting Flash of Light, Holy Light, or Divine Light on your Beacon of Light target has a 40% chance to grant you 2% of your base mana.
4-Piece - Your Divine Light also heals a nearby injured target for 20% of the amount healed.
Paladin (protection)
2-Piece - Your Shield of the Righteous deals 20% additional damage as Fire damage.
4-Piece - Your parry chance is increased by 12% while Divine Protection is active.
Paladin (retribution)
2-Piece - Your Crusader Strike deals 15% additional damage as Fire damage over 4 sec.
4-Piece - Increases the duration of your Zealotry ability by 15 sec.
Priest (holy)
2-Piece - Your Flash Heal, Heal, and Greater Heal spells cause you to regenerate 2% of your base mana every 5 sec for 15 sec.
4-Piece - You have a chance when you cast a helpful spell to summon a Cauterizing Flame. Friendly raid and party members can use the Cauterizing Flame to be instantly healed for 4625 to 5375. Lasts for 45 sec sec or 10 charges. After using the Cauterizing Flame, players cannot benefit from it again for 10 sec.
Priest (shadow)
2-Piece - Your Shadowfiend deals 20% additional damage as Fire damage and its cooldown is reduced by 75 sec.
4-Piece - While you have Shadow Word: Pain, Devouring Plague, and Vampiric Touch active on the same target you gain Dark Flames, which reduces the cooldown of Mind Blast by 3 sec.
Rogue
2-Piece - Your melee critical strikes deal 6% additional damage as Fire over 4 sec.
4-Piece - Your Tricks of the Trade ability also causes you to gain a 25% increase to one of your combat ratings at random for 30 sec.
Shaman (elemental)
2-Piece - Your offensive spells have a chance to reset the remaining cooldown on your Fire Elemental Totem.
4-Piece - Your Lava Surge talent also makes Lava Burst instant when it triggers.
Shaman (enhancement)
2-Piece - Your Lava Lash gains an additional 5% damage increase per application of Searing Flames on the target.
4-Piece - Your Stormstrike ability also increases Fire damage you deal to that target by 6%.
Shaman (restoration)
2-Piece - Your periodic healing from Riptide has a 40% chance to restore 1% of your base mana each time it heals a target.
4-Piece - Your Chain Heal spell will jump to one additional target.
Warlock
2-Piece - Your periodic damage has a chance to summon a Fiery Imp to assist you in battle for 15 sec.
4-Piece - Your Shadow Bolt and Incinerate spells have a 5% chance to increase all Shadow and Fire damage you deal by 20% for 8 sec.
Warrior (DPS)
2-Piece - Your Battle Shout and Commanding Shout abilities also grant you 3 rage every 5 sec for (unknown).
4-Piece - Your Mortal Strike and Raging Blow abilities have a 30% chance to trigger a Fiery Attack, dealing 100% instant weapon damage as Fire.
Warrior (tank)
2-Piece - Your Shield Slam deals 20% additional damage as Fire damage over 4 sec.
4-Piece - Your parry chance is increased by 6% while Shield Block is active.