So my time on Book Country has provided me with a lot of ideas for this blog. Guess I'm taking a turn towards writing advice, here. Which is fine, I mean it does say "this is a blog about reading and writing fantasy literature," so it's not as much of a digression as some of the things I've posted.
Anyway, sometimes authors use real societies as models for their fantasy worlds. A lot of fantasy is pseudo medieval European, although as Saladin Ahmed points out frequently, people don't really know what medieval Europe was like. The historical information is out there, sure, but the average person hasn't read it. Probably historians cringe when they read fantasy, like I cringe when I read science fiction with gross errors in biology and chemistry.
Anyway, sometimes authors do their research, and it shows (click here for another related post). Guy Gavriel Kay does a good job, and Carolyn Ives Gilman is an actual historian. I'd argue that some of the societies Glen Cook wrote about in the later Black Company books and/or in the Instrumentalities of the Night Books are also based on real historical societies.
But then sometimes authors just start writing, and end up with societies that are a weird conglomeration of time periods and geographical locations. And this just doesn't work. You can't have a strong, centralized church in the same place you have gladiator fights. It sounds like the author read two books, one about the Roman Empire and one about Catholic Rome, and said, "well, hey, they're both Rome" and went with it. It's like giving people in the Tokugawa Shogunate soda machines you can hug to get a soda (hey, they're both Japan) or having the ancient Greek city-states suffer through today's collective European debt crisis.
Why is this a problem? (Full disclosure, I'm not a historian, I'm a chemist.) Lots of different human societies have existed in the last couple of thousand years. But customs, languages, and cultures -- and values -- develop as coherent groups. Sure, there's exchange of ideas through travel and trade. (The Japanese written language makes use of Chinese characters, for example.) But there's no sense in throwing a bunch of ideas from a random group of human societies into a hat and then drawing out a few. And when you read, for example, about a polygamous society that has a woman in the line of succession, it's just strange. That's not the way the world works. It makes the reader think you didn't really develop a consistent set of details for your world. It confuses the reader. It makes someone like me pick apart every little aspect of your setting until there's no enjoyment left. It makes me overlook what might otherwise have been a brilliant plot, makes me think you were careless. And if you were careless when you wrote it, why should I care when I read it?