IT ONLY TAKES TWO
The list of powers gained by having just 2 enhancements of a set.

Melee Damage:

Bruising Blow	               10% Recovery
Pounding Slugfest	8% Regeneration
Hecatomb 	                4% Recovery
Gladiator's Strike
PVE + PVP	               2.5% Recovery
Blistering Cold	               10% Slow Res
Superior Blistering Cold
          	                                15% Slow Res

PBAoE Damage:

Cleaving Blow                  	1% Recovery
Eradication	                1.8% Endurance
Scirocco's Dervish	10% Regeneration
Armageddon	                4% Recovery
Fury of the Gladiator
PVE + PVP	                3.75% Fire/Cold Res, 6.25% Mez Res
Avalanche	                10% Slow Res
Superior Avalanche	15% Slow Res

Pet Damage

Commanding Presence          1.5% Recovery
Brilliant Leadership	          4% Regeneration
Edict of the Master	          6% Movement Speed
Blood Mandate	                          1.5% Recovery
Sovereign Right	          8% Regeneration
Soulbound Allegiance	          16% Regeneration

Ranged Damage

Volley Fire	                     4% Regeneration
Maelstrom's Fury                 4.5% Movement Speed
Entropic Chaos	                    10% Regeneration
Thunderstrike	                     2% Recovery
Devastation	                     12% Regeneration
Apocalypse	                     16% Regeneration
Gladiator's Javelin
PVE + PVP	                     2.5% Recovery
Winter's Bite	                     2.25% Fire/Cold Res, 3.75% Mez Res
Superior Winter's Bite        6% Fire/Cold Res, 10% Mez Res

Recharge Intensive Pets

Call to Arms	                        2.5% Recovery
Expedient Reinforcement   1.5% Pos/Neg Res, 2.5% Mez Res

Sniper Attacks

Executioner's Contract	        2% Recovery
Sting of the Manticore	        12% Regeneration

Targeted AoE Damage

Air Burst	             2% Knockback Strength
Ragnarok	             4% Recovery
Positron's Blast             2.5% Recovery
Javelin Volley
PVE + PVP                       2% Recovery
Frozen Blast                  10% Slow Res
Superior Frozen Blast
                                            15% Slow Res

Universal Damage

Overwhelming Force	    12% Regeneration

Archetype Sets*
(These appear to need 3 slots for the best bonuses!)

Blaster

Blaster's Wrath	                               7% Accuracy
Superior Blaster's Wrath	15% Accuracy
Defiant Barrage	                2% Damage
Superior Defiant Barrage             4% Damage

Brute

Superior Brute's Fury	                  3% Health
Superior Unrelenting Fury	4% Damage

Controller

Superior Will of the Controller	        4% Damage
Superior Overpowering Presence    8% Control Duration

Corruptor

Superior Malice of the Corruptor	3% Health
Superior Scourging Blast	                3.6% Endurance

Defender

Superior Defender's Bastion	3% Health
Superior Vigilant Assault	3.6% Endurance

Dominator

Superior Ascendency of the Dominator 	4% Damage
Superior Dominating Grasp	                                4% Control Duration

Kheldian

Superior Kheldian's Grace	15% Accuracy
Superior Essence Transfer	3% Health

Mastermind

Superior Command of the Mastermind	     4% Damage
Superior Mark of Supremacy	                     10% Recharge Time

Scrapper

Superior Scrapper's Strike	3% Health
Superior Critical Strikes	4% Recovery

Soldiers of Arachnos

Superior Dominion of Arachnos	 15% Accuracy
Superior Spider's Bite	                                  4% Damage

Stalker

Superior Stalker's Guile	15% Accuracy
Superior Assassin's Mark	4% Recovery

Tanker

Superior Might of the Tanker	4% Damage
Superior Gauntleted Fist	3.6% Endurance

Accurate Healing

Theft of Essence	10% Regeneration

Healing

Triage	                                         4% Regeneration
Miracle	                                        2.5% Recovery
Numina's Convalescence     12% Regeneration
Panacea
PVE + PVP	                        2.5% Recovery

Defense

Serendipity	                 4% Regeneration
Gift of the Ancients	 2% Recovery
Luck of the Gambler	10% Regeneration
Shield Wall
PvE and PVP	                10% Regeneration

Resist Damage

Impervium Armor	2.5% Recovery
Unbreakable Guard	2.5% Endurance Discount
Gladiator's Armor
PVE + PVP	                2.5% Recovery

Mez Sets

Confuse
Perplex	                   6% Regeneration
Malaise's Illusions	   2.5% Recovery
Coercive Persuasion	   7% Recovery

Fear

Unspeakable Terror	   2.2% Fear Dur
Glimpse of the Abyss       2.75% Fear Dur

Holds

Lockdown         	                      3% Damage
Unbreakable Constraint     4% Recovery
Gladiator's Net
PvE and PvP	                      2.5% Hold
Essence of Curare	      1.5% Toxic/Psionic Res, 2.5% Mez Res

Immobilize

Debilitative Action	   4% Immob Dur
Trap of the Hunter	   5% Immob Dur
Gravitational Anchor	   4% Recovery

Knockback

Volley of Velocity	6% Regeneration
Explosive Strike	1.5% Damage
Force Feedback	7.5% Movement Speed

Sleep

Induced Coma	                  CO*(Chance of various percentanges) Sleep Duration
Fortunata Hypnosis	  CO + Recovery

Stuns

Absolute Amazement	  CO + Recovery
Rope A Dope	                  CO + Regeneration

Taunt

Annoyance	                     4% Regeneration
Mocking Beratement	     1.8% Endurance
Perfect Zinger	                     2.25% Toxic/Psionic Res, 3.75 Mez Res

Flight

Soaring	      CO+ Recovery
Freebird    CO+ Regeneration