I've ran a HUGE amounts of pug. While levelling my tank, I had the bright idea that learning from *bad* groups would be better than learning from good ones and to an extent, I was right. I've also gotten to see some signs of when a group will be BAD:
. Getting a /w: "what's your hp". I'd like to see the healer trying to heal me in 15k hp with something like 30% total avoidance. People should learn the intricacies of tanking or just stop asking that idiotic question.
. PVP gear=90% of the times BAD player. More interested in topping the dps chart than managing aggro. Worst of all? PVP frost mages.
. No Omen or KTM. Sorry, but if you're serious about the game, INSTALL THE ADDON! I don't refuse to run with someone who doesn't have it, but it takes a VERY good player to manage aggro without one and, let's face it, most good players have omen installed. If you think you're in the 0.01% that can manage aggro without it, you're probably wrong.
. Greens. I'm sorry, but although you probably can output good dps / heal competently in greens, what you're showing is that you just cannot be bothered to run NORMAL instances to get equipped, skipping a step in game progression.

And of course... phrases like "omg dood l2tank wtf I always sheep-pull lol".

That's my rule for PuGs with my lock also. If I have more health than the tank, I am gone.

The other that grinds my gears is the lack of threat meters. I don't care if anyone else in the party has it, the tank needs to have it. This is especially important if there isn't a pally in the group. DPS needs to dish the damage out but if we don't know where that is between laying the smack down and laying on the ground, we can't do our job effectively. I've had so many bad experiences with sub par tanks, I will literally wait until I see 4k threat built up before I even cast a spell, unless I know the tank well.

Heroics are more dependent on good tanks and healers. The DPS side is more forgiving. But if the tank can't hold aggro and the healer can't keep the tank vertical then its a lost cause.

This isn't to say DPS have a free ride. They need to manage their aggro, keep the CC up and follow the kill order. Dumbasses like destruction spec locks who spam searing pain get kicked after the first pull, hunters who want to use scatter shot or forget to turn growl off or mages who think its funny to burn you down when you get MC'd get kicked after the first pull they do that.