I personally don't really care that much if one of my PCs use a power that nerfs an enemy; they need it with what I throw at them. However, there are some powers that I felt bad about even as a player sometimes:
Certain Justice - Champion of Order level 11 attack (whose name I forget): Marked target dazed + weakened until not marked. Really neuters solos/elites.
Solar Wrath - Radiant Servant 11 attack: used to wipe out whole encounters worth of minions and stun a couple fights full of demons, haven't seen it since it was nerfed. Almost every time our cleric used it, the DM would say "that's an encounter power?"
Lilting Songblade's item daily power: All allies get +4 to damage. Any time an enemy dies, add +2 to said damage bonus. In a fight vs mixed minions and regular creatures, at the end of the combat, we had something like +24 to damage rolls...
Knockout Strike - Rogue level 9 Daily: Our daggermaster saves this for bosses. KO + AP + another daily(coup de grace auto-crit), triggering daggermaster's Critical Opportunity attack power = a crap ton of damage(I've seen 250+ from the combo in mid-paragon when he crit with Knockout or Critical Opportunity as well).
Howling Strike - Barbarian Level 1 At-will: It's almost absurd how much damage the group's barbarian does with this thing when combined with all the charge gear. At level 11, it was something like 2d6(Brutal 2) + 3d6 + 1d8 + 15(add another 7 if he's got his Obsidian Steed out)
Sword Burst - Swordmage level 1 at-will: On its own, it's not too bad, but when combined with White Lotus Riposte/Master Riposte, Arcane Admixture(Thunder) + Echoes of Thunder + Aegis... enemy takes -3 to hit (Mark of Warding) and risks another attack(or nerfed damage if Shielding Swordmage) if he doesn't attack the Swordmage, ~4-10 auto damage(level dependent) and a repeat of Sword Burst if the enemy does attack the Swordmage. This is especially great when used in the middle of big piles of enemies (was even better before they nerfed Enlarge Spell).
Oh, also Astral Seal before it got nerfed.