If you observe the chat channels in more or less popular areas and are privy to some guild chat as well you will often find that people tell people they're doing something wrong but they never seem to make time to tell them what exactly they did incorrectly and why it is wrong to do it.
Stratagems deviced by players are often based upon experience. If you spent your lifetime blasting critters to hell as quickly as possible in solo play then that quickly becomes your preferred strategy because that is what is required to survive in PVE solo play.
However front loading Damage as a DPS class in a group situation can have dramatic consequences because whomever can take the hits doesn't have enough time to really make a critter angry.
More often than not accusations will be flung back and forth between the various members of the group resulting in generally ill-feelings towards each other and re-enforcing the idea that pick-up groups are a terrible place to be.
Fact of the matter is that simply yelling at the mage to hold the damage means nothing to the mage if he or she doesn't understand why she/he has to do this.
Granted, this is an oversimplified example and most people know why not to put too much damabe on a monster before the tank has built up enough threat but it applies to a lot of scenarios where 1 person simply doesn't have enough knowledge about another class to make the distinction of what to do and what not to do.
After all if the warrior is supposed to tank the unitiated will also assume that the warrior has some kind of ability to build threat in a hurry.
By not explaining why something is the way it is you enforce a negative feedback cycle that will only result in anger and resentment. Either spend the time preparing your group for an encounter or be prepared to explain why some things have to go in a certain order.
Worse than that it puts people in an ackward position where they simply have no clue what to do next. "So if I am to hold the damage then when can I start doing damage again and what the hell am I supposed to do in the meantime?"
I myself have been severely turned off to grouping up with other players due to the lack of appropriate feedback. If you can't tell me why it is wrong then how can you expect me to improve?