I'm usually pretty lucky with my dungeon finder groups, but every so often you run into some real stinkers. And by that, I don't mean incompetent players -- just mean ones. This was brought home to me on a recent Halls of Stone run I was healing when one of our DPS, a moonkin, got mind-controlled by a Dark Rune Controller and then killed by our mage and rogue. He or she very politely asked the other two DPS if they could interrupt the Domination cast next time to avoid that happening, and in return got an avalanche of abuse. The tank took their side while I was resurrecting the moonkin, and it was then that I noticed that all three were from the same realm.
They were still making snide comments about it while we were pulling trash to the Maiden of Grief, and I finally snapped, "He's right. You can spare a second to interrupt the cast, and if you won't do that, the least you can do is avoid killing a mind-controlled player." The abuse rerouted in my direction for a few minutes, but one of the nice things about working here is that after you've been a target of the professional trolling class, their amateur counterparts don't have that much effect.
It leaves me wondering how you deal with unpleasant players without causing a blow-up or turning into a bit of an ass yourself (as I admit I probably was while coming to the defense of the moonkin). We all run into them eventually, and I don't know whether it's better to speak up on behalf of a bullied player who may not want the help, simply drop group or spend the run in silence, not rising to the bait.