Compare this to encounters where the primary difficulty is role-specific or even player-specific. Good DPSers pushed their output to the limit on Patchwerk, healers learned to anticipate damage during Malygos' Vortex while one or two people got good at yanking sparks into the raid, and tanks grew experienced with fast pick-ups on Kael'thas. But the average raid group, even when experienced, probably tripped over and over again on encounters like Teron Gorefiend or Anub'arak. When you can't control who gets targeted by Shadow of Death or Anub'arak's spikes and when the randomness limits the experience that any one player can get ... Well, it's easy to see how certain fights acquire the nightmare moniker.
When you think about it, most of the truly difficult boss fights in the game from classic through present have all incorporated that Tag! You're it! element, and the inability to control for that randomness keeps these fights unpredictable and uncertain until players outlevel the content. As Klepsakovic observes, it's not fun for the raid group as a whole to die to one person's mistake, but it's an unfortunate method of making an encounter challenging -- raising the cost of failure.
The Raid Finder version of Dragon Soul seems like an oblique confirmation of what Klepsakovic's saying here, I think, and it's going to be interesting to see what Blizzard yanks or preserves in future encounters to keep the content accessible to as many players as possible. The true distinction between the average, the good, and the great won't be high DPS, careful gearing, or longevity -- it'll be the ability to cope with random elements that will never be present in the Raid Finder version of an encounter.