I hate derps. Not noobs that try to improve and take advice but pure derps, morons who join groups with horrible builds or good builds but for the wrong role (like tanks joining groups as dps or viceversa), having horrible gear that isn't meant for their class nor even role, etc etc, and they also play like derps, like the above tank that joins as dps, stays in defensive stance or taunts mobs off the actual tank that joined as tank.
And ofcourse when you try to tell them what to do, you hit a wall of silence as the derp probably doesn't even understand english and continues to do his thing. At this point you wonder if he's not really some kind of dungeon farming bot, or some 6 year old playing on his brother's account.
Btw, don't start that whole "privacy" thing. If you're such a...private person, stick to single player games. When you're in a group with other people, they need and deserve to know who is the poor player in the group due to whom the group keeps wiping and why. Perhaps that person can even get some tips about gearing or speccing on the spot, which he can learn and improve himself on the spot instead of not knowing what he does wrong.