Things that I have come to hate about the puggers that I seem to get stuck with.
1)When I mark, follow it.
There are 2 reasons that I mark targets. I will mark for tactical reasons, such as healing targets or caster mobs. I will mark targets to help my threat generation.
I play a Bear tank, and we don't have the best AOE threat generation, it's slow to generate and requires constant Swipe spam. So I tend to mark a first kill target so that I can generate some AOE threat before the DPS start to unload their massive AOE damage. This way, I won't lose control of the targets. I can handle their single target threat easily. So when I mark a skull target, KILL IT FIRST. It seems like every Ret Paladin seems to have a complete and utter disregard for my marking. If I mark a Skull target, they will run up and deliberately attack some other target. Please, just kill skull first, things will go much more smoothly.
2)Don't ever pull for me.
If you deliberately pull a set of mobs, I will let them beat on you, and I will pray for your death. If the healer decides to keep you alive and heal you, I will pull them off of you for the healer's sake. I am an incredible asshole in this regard. After the pack is dead, I will call you out on this.
I know that tanks aren't some amazing gift from the cosmos to save you, but just as healers are expected to heal, and I am expected to keep all of the mobs attacking me, you are expected to kill them w/o aggroing them. This brings me to number 3.
3)Keep in mind of where you are, and where the patrols are.
If you are a caster, you can't just stop moving once you get in range of the mobs. There are patrols that will move by us and latch onto you. If you are melee, keep in mind that running behind a mob might aggro another pack. I have to pick up this new pack, because if I don't, the healer will get distracted, and I might die.
Doing this is a bad thing. Yes, I can often pick up this new pack and with the use of cooldowns and some sweat from the healer, we'll make it. However, I don't want to do this, it stresses me out.
4)Know your class
My best friend was the Paladin who used Repentance on the appropriate mob without me asking. My worst nightmare was when I was on my 77 priest healing a PUG Gundrak and the DeathKnight tank didn't know what a 'defensive cooldown' was. Actually, this happened twice. I asked them to use their defensive cooldowns so I wouldn't go OOM on trash pulls and that was how they replied. Granted, I believe it later turned out that he was 11.
A basic understanding of all of your abilities and what they are useful for is a must. Next time you are leveling a character, and you obtain a new ability, ask yourself, ""What is this good for? When should it be used?" You'll find that you will be a much better player.
So please, please understand what you're supposed to do, and where you're supposed to be. It's not that hard, I promise.