I spent many years as an Interviewer/Interrogator and understanding human nature was a requirement of my job. So a few observations directed at no person in particular.
The first is that all the post above mine have some truth in them. Some more than others. I think Carl put it best.
Second; The human animal has a need to be right and to be validated in their opinion. What do I mean by this?
Well, look at the grading topics. Lets us say that a particular coin is being graded in the XF range and one of the people who are looked at as authorities gives their opinion on the grade and it might be higher or lower than the consensus before that. Almost always; all the posters after this person will post grades in line with the "authority". Don't get me wrong, that person might well be an authority. Likewise it may be a person who post alot and had a high percentage of "correct" answers.
In the above example; people are loath to "get it wrong" and will give the "consensus" answer rather than their own initial thoughts. It is human nature. Humans are, by and large, herd animals. It is very very true that individuals exist.That is how we get our Daniel Boones, Simon Kentons, John Smiths, et al. These, though, are the outliers, not the norm.
It is also a true that "everyone" thinks that "they" are the exception to the rule. Two examples. In talking about relationships you can tell a woman that "with your man, what you see is what you get. That only one in a million men ( I'm exaggerating) will ever change, and your guy will always be the same". They will almost always agree with you...but will have the "certain" knowledge that their guy is that "one in a million". If I told you I had an IQ of 147, and that in a random sampling of 100k people, that statistically, there will only be one person smarter than me; most people will assume that to be true, as well as assuming that they are that one person.
What does this mean? In our example someone is asked a grade, they see an "authority" give a statement. The fear of being wrong will subconscously lead them to agree with the "authority". The tendency to believe that they are the exception will mentally place them with the "authority". The agreement with the grade, or opinion, validating their own status as an "authority".
I think this causes problems in a couple of ways; especially when looking at errors.
Lets look at DDO/Rs. In my opinion the biggest problem we have, other than bad photography, is that a true DDO/R can have MD as well. MD occures on all coins, whether real errors or just normal coins. When one is asked to state ones opinon on whether a particular coin is a DD or not, and one sees MD, it seems the default is to say that it is "just" MD. The fear of being wrong; the need to be an authority; and the subconscious associations with "known authorities" all contribute to a certain shyness when being asked to "certify a DD".
Years ago I published some photos of some of my more obsure DDOs. Most were
LWC. At the time there was a member who was well known as an authority on
LWC, deservedly I think. The first one I posted, a 1941P-DDO1 had MD. The authority immediatly identified it as such and that set the tone for my next four coins. All of which were soundly rebuked as MD. In frustration I posted a pic of a
Jefferson nickel with a doubled profile, and it was almost attacked. One member telling me to stop posting till I learned to differentiate between MD and DD. When I revealed that the pic was lifted directly from the Variety Vista website, and posted a link; I received zero comments. Nor did any of my further postings of DD receive more than the occasional comment.
You see, I had been labled by the group as a "non-authority". That labeling prejudged every subsequent post. It did not matter that I later had a
TPG authenticate some of my DDO(s). Everyone "knew" I did not know what I was talking about. When the detractors were faced with the fact that they had been wrong on the Jefferson, not one appologized...because none of them wanted to admit publically, that they really were not an authority.
This of course was years ago and the group membership has changed to include lots of new people.
Most people are rather nice in telling someone that their coin is not an error, or is not rare or valuble. Occasionally someone is rather rude and/or dismissive.
Some people give opinions based on poor photography, though I think that the majority do indeed state that an opinion can not be made on the basis of the current photography. Unfortunately sometimes this person is an "authority", and subsequent opinions are "bandwagon" opinions...in my professional opinion.
I think this contributes, along with the things that they other posters have alluded to, disgruntled members.
I hope this in some way helps