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".