For some reason this really bugs me.
Was in a PUG last night on my mid level pure rogue (yes, I know, a PUG, you never know what you are going to get).
There was another pure rogue in the group that was three levels below me, although he leveled partway through the quest chain and was then two levels below me.
He did a few things that I wasn't that fond of, but I don't want to derail things here.
What really got on my nerves was that on three occasions, while we were fighting either a horde of mobs or a boss, there was the creaking sound of a chest opening and in chat you could see him looting. On one of these occasions, one of our party members was incapped at the time. One of the times, I had drawn aggro which is rough for a rogue, and was jumping around like a sorc in a firewall chugging pots trying to stay alive.
Each time he did this I told him that was bad form. He did not respond, nor did anyone else in party say anything.
At the end of the first part of the chain (Sorrowdusk) he dropped group without a word. The party leader wondered aloud if he dropped because of being called on looting while others were dying, and that if so that was fine because it is really bad form to do that. The rest of the party agreed, although nobody had said anything to him at the time.
My point is this. Many of us from time to time do things that others may find to be irritating, or even damaging to the party or the chances of finishing the quest. Too many of us don't say anything about it for whatever reason. That should change.
Be polite, but be firm. It may be that they just don't know, or haven't considered how their actions may look to others. If nobody says anything to them, they will never change.
I have run with a few people who are very good at this. They are nice about it, but don't tolerate garbage. IMO, that makes this game better. It's a learning experience for somebody. Either the person learns that it bothers people when they loot while the rest of the party is fighting, or the rest of the party learns something about the looter.