I don’t know about you, but I have a personal policy never to set any of my stories in the real world (or at least nowhere that could be found on a map) if I can help it. Perhaps it’s my own alienation from the world at large that inspires me to do it, but I just feel that I risk alienating the reader if I start to talk about a place that they’re not familiar with and/or do not identify with in the slightest. I mean, that’s obviously also a risk with fictional settings, but at least everyone goes in with more or less the same level of familiarity (unless you flub the effort by including comparisons to real-world elements many people would not be familiar with, have one or more characters come from the real world, etc.), and it’s the writer’s job to make sure that everyone understands all that is necessary to tell the story in an effective way and hopefully immerse the reader into the universe they’ve created.
As I have said, there are some times when the author of a piece of fiction sets it in the real world, but does it in such a way that the reader does not feel left out of the loop without having to peruse an abundance of footnotes. That’s always admirable, but unless it deals with certain aspects of humanity or nature or human nature that are underaddressed even today, I much rather prefer keeping it as fictional as possible.
Are settings the only elements of fiction that can have this restriction? Of course not. If it exists in the real world and you’re willing to assume I already know about it, you can alienate me with it. Ideologies, societies, doctrines, and many other abstract elements can be lifted from the real world as well as a location, and unless handled well, that might bring narrative disaster about.
In my opinion, one of the most effective ways you can go about including something like this short of dropping an anvil would be via those wonderful inventions known as allegories. If you tell a story that has a deeper meaning to it that’s never spelled out over the course of the text, that seems like a fantastic way to get a message across without messing up the flow of the story. Like many things, this can also go wrong, and you may end up creating something that students of language and literature will be puzzling over for centuries (something I also try to avoid), but if you believe you have enough skill to get a message across subtly without making it obtuse, then it seems like the best option.
As for you, where are you willing to draw the limits between where reality ends and your stories become completely fictional? If you feel there is something you have on your mind relevant to the topic at hand that merits saying, the comment box is just down there.