First of all, there is no given roll that should always be blamed, and most of the time, no one should have to be. If someone makes a mistake, try to tell them in a constructive way, give them advice. If someone criticizes you, listen to what he/she says, and then if it sounds like good advice, keep it in mind. If it doesn't, try telling whoever told it to you in a nice way. This should actually be pretty easy, as long as everyone keeps in mind that the rest of your group consists of real people that should be treated with respect. Keep in mind that some of these people are still in an early stage of the wow learning process, while at the same time, people should learn how to accept and take in the advice they recieve, you do not have to see every comment suggesting improvements as an isult Remember, the only bad players are players who refuse to improve. Not only is this great for improving your gameplay, but also for getting the most enjoyment out of playing.

When you meet one of those players who just won't listen to constructive criticism, what to do?
Sometimes, even if you are nice and patient, you will happen to meet some people who just refuse to listen to advice or change their behavior, for example someone who makes a mistake and then start flaming others instead of listening to what the group has to say. Sad to say so, but if this is the case, kicking that player might be the best choice, running with rude people often ruins the game experience quite a bit.

The group leader.
Even though there's only one player in a group being the leader, you are all still members of a team and should work together, also, no specific role is always to be the unofficial or official leader of the group. I'm mostly refering to the idea that a tank should always be leading the group, wich he/she should sence it's the tank that starts a pull and the group follows his/her pace. Just keep in mind that even tanks can be unfamiliar with a dungeon.
As a group leader you do get some extra privileges, just keep in mind that with privileges comes responsibility and you should still keep the first part of this guide in mind.

In shorter words, being in a dungeon group with random people shouldn't be harder than waiting for the buss with random people irl. Just be nice and patient and when someone starts flaming you or others, don't just sit there and let them keep doing so, but tell them without overreacting. (For example starting to flame that person or send loads of angry letters)

I know this may not seem like an important guide to some people, but seeing the number of threads conserning this matter that I have seen, I thought I'd share my philosophy. Hope it helps!