The players need to understand that while they take part in the world,the DM is *creating* it, making it come alive.
I liken it to writing a book, you need to write about material that you are comfortable with. If you normally write modern mysteries, you may have a hard time doing a story with magic and elves.
Even a science fiction author may specialize in space battles, and another in various race interactions. If William Gibson is creating a Cyberpunk-type world in a book, and his colleague asks him to "add in this exterestial race with 4 tentecles"; he will not be as good at it, will not be comfortable with it, and his story (or the DM's world) will suffer because of it.
It is the DM's responsibility to try and make it fun for the players also, and that includes allowing things they find fun, even if you don't 'like' what they are adding. But this does *not* extend to adding things that will not work well in your world. Personally, I would have a very tough time creating a living world that was at all believable and consistent, if it was very high magic. I just can't make it work. So I dont allow those types of additions.
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