We've all seen the threads and threads of discussion about bad PuGs, bad heroics, tanks being jerks, dps being morons, healers refusing to heal, and all that jazz.
But, what actually makes a random heroic fail? What makes that VoA run so bad that it just can't finish, or down Toravon? What makes that raid weekly not able to be completed, even though it's the easiest boss on earth?
Pick your top 3.
Personally, I think Strategy, Healing Inability, and DPS Inability are the big 3 killers. People simply not knowing the fights are too much of a detriment to ignore on anything harder than Naxx, Healing can be flaky (especially in PuGs) and DPS, no matter how much you check gearscore, is always a little too shaky to rely on.
How about everyone else?