The biggest thing I learned in PUGs is to be specific. Assume nothing. The other night I was in a half PUG (I had a healer who is a good friend and one of his guild warlocks) with a random DK and ele shammy.

We were in H Throne of the Tides and on the first boss I gave the fateful instructions: "Burn the melee adds and cc the casters." The warlock offered to CC the left add, the shaman said he'd take the right. Okay.

Boss comes out, we're avoiding the floor-death, and since I had never tanked this boss before I was tanking her in the middle. Apparently, the melee add comes out closer to the back of the room, so the DK was way closer to her. He took to heart the words "burn" and immediately started in on the add, getting two-shot in the process. We then wiped due to not having the DPS to burn the adds before she came back up.

What I learned? Be specific. I quickly added to my instructions that "When I say burn, 'let the bear hit it first' is implied." But nothing is really implied. If you mean you'll be tanking something but they need to DPS it down quickly, say so. Especially because on that last boss in the dungeon, I would not be tanking the murlocs or channelers, but they still needed to be burned down hard.

You cannot be specific enough, because no one knows each others' playstyles very well, you have to kind of learn on the fly that the DK will be helpful and death grip casters or that the ele shaman does not have a clue what a LOS pull is.