The Dunning-Kruger effect is summed up pretty easily -- people with low skill (or "competence") don't realize that they have low skill. They don't have the ability to understand that they're not as good as everyone else. (This can be fixed by raising their competence level, which will allow them to recognize that they were previously incompetent, but until that time they are unable to recognize skill in others or lack of skill in themselves.)
People who have high skill (or "competence") tend to think that they have average skill because they assume other people have "equivilant understanding." That is, they think other people can grasp the same things they do.
Thus, people who are utterly incompetent believe themselves to be average and people who are very competent tend to put themselves just slightly above average.
I've seen this phenomenon in raiding.
(This is the part that might get me in trouble.)
The incompetent raiders don't get rostered quite as much, especially if a raid is over-filled on a certain class (I've heard warlocks have been a rostering issue in our 10-mans). When one of the warlocks complained about not getting rostered, he quoted another warlock as saying "I guess you have to be sleeping with the guild leader to get rostered around here."
This was a pointed attack (probably misquoted, because The Big-Mouthed Warlock is famous for misrepresenting people) at my sister-in-law. My brother is standing in as guild leader and he and his wife were, at the time, rostered every raid.
Because they were healers.
Because they were the most experienced healers.
Because the guy doing the rosters only liked raiding with them because they were highly competent experienced healers.
When I praise my brother and sister-in-law for their healing abilities, I am not doing so out of favoritism or affection.
The pair can heal.
They are highly effective, communicate well with each other, and get stuff done. Either one of them can keep up idiot overpulling tanks in pugs and have, on more than one occasion, kept up a 10-man that went to crap. My sister-in-law pulls spells out of her arsenal that most people don't even know exist.
They are very VERY VERY competent.
I think it was the Dunning-Kruger effect that made the warlocks complain. They couldn't realize that their raiding competence wasn't anywhere near my brother and his wife, and that (along with additional practical reasons) that was why they didn't get rostered as much.
Yet they complained.
Getting rostered more than them.
It's mind-boggling, isn't it? But the Dunning-Kruger effect clearly states that incompetent people will boggle your mind. Because they just don't get it. And they won't get it because they can't. Their very incompetence prevents them from recognizing their incompetence.
So. That's why incompetent raiders get cranky and don't understand why you don't roster them more. Ta da! I solve your conundrums.