Let's get some ground conditions.
First, remember that this is a game. People play games differently.
My dad tends to play board games ruthlessly, taking ten minutes per turn (this is why we add time limits to Scrabble). My mom is less focused on only winning and cares more about having fun. Also when we were little kids my dad would be the one taking all our money in poker while my mom would scold him.
Getting back to WoW: it's a game. This is not a get out of jail free card though. After all, you'd be expected to know how to dribble or shoot in basketball. Still, no one expects you to be NBA quality for a pick up game in the park. There are expected levels of understanding but low standards of skill, since it is a game after all. Neighborhood leagues are likely to have higher standards, but the focus remains on fun. As you get higher up the focus shifts more towards skill. This creates an elite, but not elitists. Can you imagine the uproar that would develop if a NBA star spent half his free time mocking people playing in driveways? He wouldn't look better for it, he'd just look like a jackass. How is WoW different? Well, for one even fewer people are paid to play, so elitism looks even more silly.
Getting back to other players
The odds are that someone else will screw up. It's simple math, out of 25 people of equal skill, you only have a 4% chance of being the one who screws up at a given time. This ranges from low DPS to bad healing, the possibilities are endless. In a 5 man that goes up to 20%. But the end result is that other people are far more likely to screw up. This is a simple reality of the world: other people are far more likely to do anything just by virtue of there being a lot more of them (you're outnumbered six billion to one).
Let's say you're far better, 10 times as good. Now in a 5 man there's closer to a 24.4% chance that the screwup will be someone else (you're down to 2.4%). Now it's clear that the reason you're screwing up less is skill, but be honest, are you really 10x better than everyone else in that group? Being 10x better in a 10-man will create a smaller difference in the screwup chance and even less in a 25-man. Eventually you stop mattering as much and your skill isn't what gets you by, it's the sheer numbers of other people.
What I'm trying to say is that much of the fail in other people is a matter of perception and your inability to do basic math. Or perhaps refusal to do the math because it would hurt your ego. Or to put it another way: fourth grade math says you're not as e-awesome as you think.
Let's move on to ignorance now: There is a lot that people don't know and don't know how to know. Huh? Imagine you've just been told "learn2play, find a DPS rotation." First off, note that this is much more helpful than usual, since it include the key words "DPS rotation." That's something you can look for much more successfully than trying to Google "learn2play." Now go find one.
"paladin DPS rotation" Let's see: first one is wowwiki. Quotes from elitist jerks and a priority system. Well that was easy. Scrolling down... oh, alternate rotations. Which one is better? I'll go with the fist one I saw, let's see now, wtf does proc mean? Seems to be something about activating, I guess proc is good. Still not totally clear if I want to use SoB or SoC, it said SoB is preferred, but then it goes on about SoC weapons. But why am I taking advice from a wiki with sources by "elitist jerks?" I'm not sure I like the sound of that source.
"Paladin damage" returns a nearly useless search, with mentions of spell damage, apparently 1-2 years old. Here's a fun one: "A Mageadin is a Paladin with +DMG armor on. You might think this is very stupid but you are wrong." That's the first result.
I'm not going to claim that information is hard to find, but we're not exactly at the point when we can psychically search all information known to mankind and distill it to exactly what we need and nothing more or less with the help of a powerful AI and super-intelligent aliens.
If they're a moron, you're a slacker
Every time you call someone a noob and kick them or leave the group, you're creating one more person who isn't quite sure what they're doing, but only jerks seem to say anything, so maybe they're doing just fine and those people are just jerks. In other words: when you are an asshole and don't help people, they don't end up any more skilled, in fact they may only become more sure of themselves in reaction to you being an asshole.
But is it your job to train PUGs? Not exactly, unless you like 'living' in a world that people don't fully understand. Jobs have schools and training, so it's perfectly acceptable to expect someone to know what to do before they get there (excluding company-specific info). Basketball you were probably taught by your dad or older siblings or maybe a local kids team. Again, it's understable that you'd be expected to have some clue what you're doing. There are no schools or siblings in WoW. I came in as a noob and I stayed there for quite a while. I learned slowly, never being taught much of anything by anyway. How many PUGs did I harm because no one pointed out any problems or made any suggestions? I don't know, but I'm sure it's a non-zero number.
You don't have to teach anyone anything and you definitely don't have to hold anyone's hand or put up with true idiots. However in the long run you're better served by the current idiot becoming decent and having a memory of someone helping. That just might start a trend that helps improve the majority.
Wrapping it up
Math says you're not as good as you think and by the reverse, that others are not as bad as you think. So poke your balloon head with a pin and let out some of that hot air.
Ignorance is curable and the recovery time is much shorter with some very basic help. Note how useful a simple phrase like "DPS rotation" was for turning an insult into something slightly helpful.
Next time you complain about noobs, remember that you're probably doing nothing to help the situation. If you expect them to learn all on their own, don't expect them to be up to your standards any time soon. Ultimately you only hurt yourself by being a jackass.
Thea: The Awakening
8 hours ago