Everything is subject to change, this is an "as is" comparison. Disclaimer, I do not have extensive knowledge on classes and specs other than Retribution Paladin. This thread is for discussion on a dps focused thread opposed to a mixed "T20" thread.
Frost Death Knight:
Item - Death Knight T20 Frost 2P Bonus (New) Empower Rune Weapon increases your damage dealt by 20% for 10 sec.
Item - Death Knight T20 Frost 4P Bonus (New) Reduce the cooldown of Empower Rune Weapon by 1 second for every 60 Runic Power spent.
Unholy Death Knight:
Item - Death Knight T20 Unholy 2P Bonus (New) Death Coil causes one of your Army of the Dead ghouls to explode, dealing [ 150% of AP ] damage to up to 0 nearby enemies.
Item - Death Knight T20 Unholy 4P Bonus (New) Each rune you spend reduces the cooldown of Army of the Dead by 3.0 seconds.
Havoc Demon Hunter:
Item - Demon Hunter T20 Havoc 2P Bonus (New) Blade Dance refunds 40 Fury when it hits an enemy.
Item - Demon Hunter T20 Havoc 4P Bonus (New) Blade Dance's critical Strike chance is increased by 50%.
Feral Druid:
Item - Druid T20 Feral 2P Bonus (New) Gain 2 Energy each time Rip deals damage.
Item - Druid T20 Feral 4P Bonus (New) Rip deals 15% increased damage and lasts 8 sec longer.
Survival Hunter:
Item - Hunter T20 Survival 2P Bonus (New) Lacerate deals 15% increased damage and Flanking Strike increases the duration of your Lacerate by 6 sec.
Item - Hunter T20 Survival 4P Bonus (New) Mongoose Bite consumes 3.0 seconds of your Lacerate on your target, instantly dealing 3.0 seconds worth of remaining damage.
Windwalker Monk:
Item - Monk T20 Windwalker 2P Bonus (New) Rising Sun Kick applies Rising Fist to the target for 8 sec, increasing the critical strike chance of your Fists of Fury against the target by 25%.
Item - Monk T20 Windwalker 4P Bonus (New) Fists of Fury increases the critical strike chance of your next Rising Sun Kick by 50%.
Retribution Paladin:
Item - Paladin T20 Retribution 2P Bonus (New) Blade of Justice generates 1 additional Holy Power.
Item - Paladin T20 Retribution 4P Bonus (New) Judgment also increases the damage of Blade of Justice.
Assassination Rogue:
Item - Rogue T20 Assassination 2P Bonus (New) Garrote's cooldown is reduced by 6.0 sec.
Item - Rogue T20 Assassination 4P Bonus (New) Garrote generates 1 additional combo point and deals 20% increased damage.
Outlaw Rogue:
Item - Rogue T20 Outlaw 2P Bonus (New) Free Pistol Shots increase your critical strike chance by 10% for 8 seconds.
Item - Rogue T20 Outlaw 4P Bonus (New) Adrenaline Rush persists for 15 sec longer at half power.
Subtlety Rogue:
Item - Rogue T20 Subtlety 2P Bonus (New) Symbols of Death costs 30 less Energy.
Item - Rogue T20 Subtlety 4P Bonus (New) Symbols of Death causes your next 3 Shadowstrikes to critically strike.
Enhancement Shaman:
Item - Shaman T20 Enhancement 2P Bonus (New) Crash Lightning increases your critical strike chance by 5% for 16 sec.
Item - Shaman T20 Enhancement 4P Bonus (New) Stormstrike critical strikes increase the damage of your next Crash Lightning by 30%, stacking up to 10 times.
Arms Warrior:
Item - Warrior T20 Arms 2P Bonus (New) Colossus Smash reduces the cooldown of Ravager by 1.2 sec.
Item - Warrior T20 Arms 4P Bonus (New) While Ravager is active, you automatically cast a total of 4 Mortal Strikes at random nearby enemies.
Fury Warrior:
Item - Warrior T20 Fury 2P Bonus (New) Raging Blow has an 10% chance to generate 40 additional Rage.
Item - Warrior T20 Fury 4P Bonus (New) Enrage generates 6 Rage and heals you for 3% of your maximum health over its duration.
Brief Conclusions:
Frost DK: Stronger burst and sustained dps through a steroid on Empowered Rune Weapon and a cooldown reduction mechanic via spending runic power.
Unholy DK: Fairly strong increase to single target and aoe damage and the cooldown of Army of Ghouls is being reduced. There is some rng involved in this set with which ghoul explodes however, overall it seems pretty standard and is a breath of fresh air from all of the rng in their kit.
Havoc DH: With First Blood Blade Dance is used in single target and with generate 20 fury with 2 set. 4 set increases the critical strike chance of Blade Dance by 50%. Overall, more fury generation, more single target and aoe damage. Another good showing of Havoc tier bonuses.
Feral Druid: The 2 set bonus becomes extremely strong in aoe situations where multiple targets have Rip on them. However, in single target the 2 set bonus is okay, more energy but nothing game changing. The 4set bonus makes Rip deal even more damage and last longer. This is very good for them in single target as they will do more damage over time and will give more energy over the extended duration.
Survival Hunter: I have never played this class and have no comment.
Windwalker Monk: The 2 set and 4 set focus purely on single target damage with respect to critical strike. Considering Windwalker is strong in aoe situations, this tier set is very powerful as a stimulus for single target damage. If the bonus also applies to your Spirits then aoe will also receive a fair increase in total damage.
Retribution Paladin: Our 2 set is somewhat mediocre, the one more Holy Power bonuses can be useful however, now using Blade of Justice/Divine Hammers over caps your holy power at 3. I am unsure of how this will impact total damage as it can be useful and it can be bad. Our 4 set is potentially a game changer in terms of talent selection, boosts our single target and cleave damage; overall pretty good.
Rogues: I do not play rogue and do not know how strong these bonuses are.
Enhancement Shaman: I do not play shaman and do not know how strong these bonuses are.
Arms Warrior: The 2 set is helpful in sustained aoe situations as it works to decrease the cooldown of Ravager/Bladestorm. The 4 set is amazing in single target and aoe. It provides 4 free mortal strikes which is an impressive amount of single target damage when focused on one target and just a small little give me in aoe.
Fury Warrior: Raging Blow now has a chance to provide a hunk of rage which is pretty helpful in single target and aoe situations. The 4 set is worthless, if it isnt 6 rage a second that is. If the set does provide 6 rage a second when enraged then its shaping up to be a pretty decent bonus.
Overall the Retribution set is the red headed step child of the melee classes. The set provides a minimal damage increase and may actually decrease our throughput in certain situations. How do you guys feel about the Retribution set in comparison to the others and what would you do differently?
EDIT: Overall the Retribution set seems fairly strong. Our 2 set is underwhelming as you might have even more downtime to avoid over filling our Holy Power. At the same time, if using Blade of Wrath there can be lucky procs leading to a combo string of Blade and Templars Verdicts. Our 4 set is potentially a game changer, depending on the new encounters we might see Greater Judgement become king when used with Divine Hammers. Overall our single target and cleave damage is being adequately boosted however, there are no benefits in aoe as of yet.