One of the most irritating things about raiding would have to be ninja AFK's. You wait for the results of a ready check only to see that -- oh, crap -- someone hasn't returned a verdict. Now, AFK's can happen for good reasons (children, explosions, children with explosives), but just as often they don't. On farm content, it's an irritating wait. On progression content, you check your watch and realize you're inching that much closer to a respawn timer and a raid leader who'll need a prescription for that skyrocketing blood pressure of his. As time passes, the clamor to just pull the boss rises, but that's not really possible on an achievement-oriented run like The Undying or something like a 3-drake Sartharion.
There are still a few fights where you can AFK entirely and no one would be the wiser (here's looking at you, Patchwerk), but they're a dying breed. Too Many Annas, much to our surprise, managed a successful Heigan kill with an AFK player, and Heigan's notoriously unforgiving even to the inattentive. The tactic involves a Paladin and what I assume to be a supremely annoyed heal team, but apparently it worked -- and I am mightily tempted to suggest it for our next Naxx clear.