Update 2 : Thanks to WoWinsider.com and Worldofraids.com for using this list and giving proper credits.

Wowhead was updated with sets bonuses ... Tier 6 ones.
Keep in mind that these are totally not official, and aren't from the 2.1.0 version. They are set bonuses already included in game and could (probably will) change when they definitely reach the servers with the next patch


Druid
Thunderheart Harness
2 pieces:Reduces the energy or rage cost of your Mangle ability by 5.
4 pieces:Increases the damage dealt by your Rip and Ferocious Bite abilities by 15%.

Thunderheart Regalia
2 pieces:Increases the duration of your Moonfire ability by 3 sec.
4 pieces:Increases the critical strike chance of your Starfire ability by 5%.

Thunderheart Raiment
2 pieces:Reduces the cooldown of your Swiftmend ability by 2 sec.
4 pieces:Increases the healing from your Healing Touch ability by 5%.

Hunter
Gronnstalker's Armor
2 pieces: Increases the mana you gain from your Aspect of the Viper by an additional 5% of your Intellect.
4 pieces: Increases the damage dealt by your Steady Shot ability by 10%.

Mage
Tempest Regalia
2 pieces: Increases the duration of your Evocation ability by 2 sec.
4 pieces: Increases the damage of your Fireball, Frostbolt, and Arcane Missiles abilities by 5%.

Paladin
Lightbringer Battlegear (Should be Armor according to T4 and T5 names)
2 pieces: Increases the mana gained from your Spiritual Attunement ability by 10%.
4 pieces: Increases the damage dealt by your Consecration ability by 10%.

Lightbringer Battlegear
2 pieces: Your melee attacks have a chance to grant you 50 mana.
4 pieces: Increases the damage dealt by your Hammer of Wrath ability by 10%.

Lightbringer Raiment
2 pieces: Increases the critical strike chance of your Holy Light ability by 5%.
4 pieces: Increases the healing from your Flash of Light ability by 5%.

Priest
Absolution Regalia
2 pieces: Increases the duration of your Shadow Word: Pain ability by 3 sec.
4 pieces: Increases the damage from your Mind Blast ability by 10%.

Vestments of Absolution
2 pieces: Reduces the mana cost of your Prayer of Healing ability by 10%.
4 pieces: Increases the healing from your Greater Heal ability by 5%.

Rogue
Slayer's Armor
2 pieces: Increases the haste from your Slice and Dice ability by 5%.
4 pieces: Increases the damage dealt by your Backstab, Sinister Strike, and Mutilate abilities by 6%.

Shaman
Skyshatter Harness
2 pieces:Your Earth Shock, Flame Shock, and Frost Shock abilities cost 10% less mana.
4 pieces:Whenever you use Stormstrike, you gain 70 attack power for 12 sec.

Skyshatter Raiment
2 pieces:Your Chain Heal ability costs 10% less mana.
4 pieces:Increases the amount healed by your Lesser Healing Wave ability by 5%.

Skyshatter Regalia
2 pieces:Whenever you have an air totem, an earth totem, a fire totem, and a water totem active at the same time, you gain 15 mana per 5 sec, 35 spell critical strike rating, and up to 45 spell damage.
4 pieces:Increases the damage dealt by your Lightning Bolt ability by 5%.

Warlock
Malefic Raiment (Looking for more informations on the first bonus)
2 pieces: Each time one of your Corruption, Curse of Agony, Immolate, or Unstable Affliction spells deals periodic damage, you heal XXXXs1 health.
4 pieces: Increases the damage dealt by your Shadowbolt and Incinerate abilities by 6%.

Warrior
Onslaught Armor
2 pieces: Increases the health bonus from your Commanding Shout ability by 170.
4 pieces: Increases the damage of your Shield Slam ability by 10%.

Onslaught Battlegear
2 pieces: Reduces the rage cost of your Cleave ability by 3.
4 pieces: Increases the damage of your Mortal Strike and Bloodthirst abilities by 5%.