Historians’ cardinal sins, no. 1. Commenting on a paper or reviewing a book or refereeing a manuscript and saying, in effect, “But you didn’t say what I would have said!”
This is too bloody common. There’s a type of whom I know a few examples, a very decent man whose specialty is, let’s say, transcendentalism. He has his own particular view of transcendentalism, which is wrong, but that’s fine. He also manages to think that EVERYTHING is explained by his version of transcendentalism. So at a panel on the Whig party, he’ll say “But this is about transcendentalism.” At a panel on the slave trade? “But this is about transcendentalism.” The Cold War? “But this is about transcendentalism.” You’re very nice, you know, but I wouldn’t want to sit next to you on a long airplane ride.
Or there’s the similar book review. I got two for my most recent book that were basically just like this. “This book doesn’t have enough about taxes in it.” Too jolly right, mate. Suppose it had: would it have substantially changed the argument? No, except for making it more boring. “This book doesn’t cover post-1945 history.” Amazingly enough, the book itself explains that it’s concerned with a phenomenon that first occurred in the period 1865-1914. Strange that it doesn’t wamble on about the post-’45.
Why didn’t I write the book you would have written? Because if I had, one of us would be out of a job.
Really, you should only say “but this book should have more about x” if having more about x would significantly change the argument or the story. Otherwise you’re just refocusing discussion on your pet rock.