The people who leave pug groups are usually the problem.
They tend to be really tightly wound.
Case in point: a tank who said the group wasn't fast enough (at level 64) leaves four pulls in. Same run, the dps who keeps standing in poison leaves after our third wipe on the first boss (whom we get down after he's replaced).
In a level 80 heroic, the tank overpulled and we wiped. Except that this was fine, nobody whined about it, it happens. We picked ourselves up and rezzed, and the guy pulled the mobs again. And dropped group. And what happens when you leave a group tankless in the middle of a pull? Right. The group dies. (Except for the rogue, who vanishes. That'd be me. Ta da!)
Generally, I've been really mellow about pugging with the LFG tool (even when people are jerks, the inconvenience tends to be minimal). I think it helps that you're virtually guilt-free if you want to leave, as long as you do it right.
"OMG U R SO SLOW!" is not polite. "I'm so sorry. This is taking a bit longer than I expected and I'm going to need to go" is fine. The other day, on a veeeery slow-moving pug, I left after we couldn't keep up with the waves of adds on the first boss. I said: "I'm low dps and I think it'll be better for you if I give up my spot. Good luck!"
Since I was in greens and blues and the group wasn't all that pro (the tank stopped between every little pull), I figured an uber dps could fix a lot for them and I gave them that chance. Plus, the run was getting painful.
Also if you're going to leave a group, I recommend you do it just after a wipe while everyone is running back. This will guarantee that the group can requeue to fill your spot and that the new addition can join the group at the beginning of the instance and run back with them (avoiding annoying "where r u?" scenarios).
You tend to fall into groups that are either slow or fast. The fast ones are more common in 80 heroics because raiders blow through for emblem farming and it's pretty easy to get carried. I have no illusions that I've been carried a lot on Birdfall. I try to be pro with my spell interrupts, but I'm the "low" in low-dps groups. Birdy's not a raider and she doesn't try to be. Luckily, nobody's given me a hard time yet or tried to "fix" my gear and spec. This Drama Mamas post about a hardcore giving advice to a casual made me think about it. Sure, I agree with the advice the guy got, particularly Lisa's (I'm a big fan of turning criticism into something that's still helpful but overall more palatable), but as someone who is the person who gets carried sometimes . . . we casuals (yeah, I'm casual now) can suck it up.
I'm not saying we need to put up with rude people, but if a hardcore wants to tell us what they think we should do with our class, let them. We don't actually have to take their advice, and if it makes them feel better to think they're doing the World of Warcraft a service, fine. We can listen and see if they have a point. Or we can just let them talk and not pay attention. If they're getting on your nerves, you can whisper them and ask them to dial it back a bit.
Sometimes advice is helpful. I was on an Underbog run earlier and the shaman and hunter were arguing about a piece of leather gear. I finally looked at it and said "Oh, that's spellpower. It's best for the shaman."
He said "It's leather. I wear leather."
I said "Spellpower is useless for hunters. You want attack power."
He listened and passed on it.
One last thing I want to emphasize for casuals is that you really do need to go in prepared. You may not know the boss fight, but you should at least have reagents and food. I have nothing against the DK who didn't bring reagents for his ghoul, but we really needed the extra dps. So just be prepared. K?
For hardcores, you know I love you, but I think a lot of you have forgotten what it's like to be undergeared. And I don't think you've ever known what it's like to be casual. So if you want to give someone advice, cool, but keep in mind that if we are undergeared, we probably don't have your resources or tenacity -- we can't just pick things up that you think we should. For Birdfall, LFG and Wintergrasp are the only ways she gets gear. I think the most frustrating hardcore/casual encounters are when a hardcore assumes that a casual wants to be raid-ready and gets frustrated when casuals aren't willing to go to great lengths to make that happen.
So my advice for dealing with each other? Chill out. If someone's being a moron, ignore him or give a polite excuse and drop group (without wiping everyone). The LFG tool replaces party members immediately, so there's really no reason to stay in a torturous situation if you don't want to. Relax. Have fun. Don't freak.
It'll be okay.