We can't do it.
The guild's been farming Black Temple for months and most of the usual roster is rocking at least 3 or 4 pieces of Tier 6 -- yet the vast majority of our Supremus fights end with more than half the raid taking a dirt nap. We've reached the point where even this is still usually a one-shot, but it's more than a little disturbing. How can you one-shot Illidan with 24 people still alive...and somehow fail to steamroll the giant McLootLoot?
To be fair, it's not just raids. I've seen players break out in a cold sweat over Talon King Ikiss on heroic Sethekk (which always confused me; the great secret of the fight consists of tanking him in the doorway). Others cite Grandmaster Vorpil, the event before the second boss in Blood Furnace when you don't have a Paladin tank, or Warbringer O'mrogg on heroic Shattered Halls (actually, all of Shattered Halls can pretty much bite me. I hate tanking that place). My own personal nemesis is Vexallus on heroic Magisters' Terrace. I've even taken a group with two mages and a rogue in T5/T6 here and wiped 3 times. What gives?