Since I'm only in an MA program I figured that I would let some flaired users do the talking. These are all responses to related questions. I found them by searching stuff like "professional" and "amateur".

In response to the question "What do you feel about your education is different/better than an amateur can learn? Access to source material? Class discussion? Can a self taught historian be as knowledgable as a professional?"

/u/DieMensch-Maschine said

I'll try to keep this concise. First of all, having a professional affiliation (even as a graduate student) offers a certain amount of cred when attempting to access primary documents. Most of the archives I had traveled to demanded a letter from my advisor/institution before they allowed me to put my grubby paws on their precious 300-400 year old manuscripts. In addition, there are expensive/rare secondary materials that often cannot be easily/cheaply obtained by a non-academic historian. Even when affiliated with a fourth tier institution, materials can still be ordered via interlibrary loan, all because that institution is willing to vouch for its students or faculty. The instruction offered in a collegial academic setting is perhaps the difference between a professionally trained and an amateur historian. In my experience, amateur historians are rarely capable of utilizing theory, historicizing, interrogating the source materials or taking an interdisciplinary approach. Furthermore, many cannot distinguish between peer-reviewed, credible scholarly work and an ahistorical hatchet-job that some conspiracy nut may have written with a crayon on a roll of toilet paper, citing YouTube videos as source material. In graduate school, such a diverse skill set can be acquired in a relatively efficient manner - and even then, it's still a baptism by fire with a sizable attrition rate. To undertake such a task on one's own with no guidance from a whole body professionals (not just historians, but academics across varied disciplines and specializations) seems beyond herculean. Finally, there's the benefit of networking. When I was just entering graduate school, my advisor informed me that in order to be an effective scholar in my chosen field, I would not only have to acquire a series of dead languages, I would then have to learn the appropriate paleographical skills in order unlock period manuscripts, which were located in these and these particular archives. Fortunately, my institution offered a course the said dead languages, which I could then adapt to my research via a particular summer program, the director of which was a onetime colleague of my advisor. The said director was then able to introduce me to other scholars who worked in a field that was closely related to my interest. One of these invited me to a conference, where after a successful presentation (according to my academic peers), I was invited to contribute an article to a journal she just happened to be the chief editor of. Once submitted, this article will be reviewed by other scholarly experts in my field before being published. This ability (and means) to produce credible scholarship is perhaps the ultimate difference between a professional historian and an amateur historian.

Another thread asked about William Shirer's Rise and Fall of the Third Reich and why it has been discredited by historians. I feel like parts of this answer are applicable to Diamond and Carlin, Carlin to a lesser extent. From a now deleted account:

The common complaint that I have seen, is that it is a journalist's work not that of a historians. Being that it is a journalist's work, there is a tendency to editorialize. Its not that historians don't value the views of the "man on the ground" the opposite is true, historians love when they have access to the views of people who witnessed the events firsthand. Richard J. Evans, a vocal critic of Shirer's work, cites him multiple times in his books on Nazi Germany, because Shirer was on the ground when many of these events were happening and so his first hand accounts are valuable. The problem comes when Shirer moves away from first hand accounts and adds his own analyses and his own ideas about history. He angered a number of historians, especially German ones with his retelling of German history and culture, and many historians felt he lacked a proper understanding of German history. Here is what Richard J. Evans says about the work in his book "The Coming of the Third Reich" Shirer’s book has probably sold millions of copies in the four decades or more since its appearance. It has never gone out of print and remains the first port of call for many people who want a readable general history of Nazi Germany. There are good reasons for the book’s success. Shirer was an American journalist who reported from Nazi Germany until the United States entered the war in December, 1941, and he had a journalist’s eye for the telling detail and the illuminating incident. His book is full of human interest, with many arresting quotations from the actors in the drama, and it is written with all the flair and style of a seasoned reporter’s despatches from the front. Yet it was universally panned by professional historians. The emigré German scholar Klaus Epstein spoke for many when he pointed out that Shirer’s book presented an ‘unbelievably crude’ account of German history, making it all seem to lead up inevitably to the Nazi seizure of power. It had ’glaring gaps’ in its coverage. It concentrated far too much on high politics, foreign policy and military events, and even in 1960 it was ‘in no way abreast of current scholarship dealing with the Nazi period’. Getting on for half a century later, this comment is even more justified than it was in Epstein’s day. For all its virtues, therefore, Shirer’s book cannot really deliver a history of Nazi Germany that meets the demands of the early twenty-first-century reader Shirer felt the views about his work were unjustified and assumed that it was simply a case of academics being opposed to a journalist writing history. Shirer wrote another book about the fall of France's third Republic and in the preface he thanked: a number of eminent French historians, none of whom share the disdain their American academic colleagues have for former journalists breaking into their sacred field — that stupidity is unknown in Europe So basically Shirer wrote a readable work, with a lot of valuable first hand accounts and interviews, but his understanding of history and the historical method is flawed and that is what keeps his book from being (in historian's eyes) anything more than introductory read.

Hopefully the responses, better ones than I could hope to give, are helpful to you. Basically one of the main things that distinguishes professionals from amateurs is that professionals tend to stay away from the sweeping narratives that Diamond makes, because those narratives tend to oversimplify. Generally if something seems too simple or too good to be true it, like how Diamond presents it, then it is.

One difference between a good historian and a bad one is familiarity with the primary sources. If you're familiar with a period of history for which the primary sources are very few, and you read a history of that period, it becomes immediately obvious whether or not that historian is familiar with the primary sources or whether his knowledge came from other secondary sources.