I think we should limit it to questions for which RPG experts are the ideal resource for answering the question. A question better answered by some other group, whether it's another StackExchange site or the wider Internet, doesn't belong here.
A question about the speed of water travel, such as this one, should probably be on a history site. A question about dangerous real-world animals for a no-magic setting should probably be on a biologybiology site. A question about a setting based on novels/movies/etc, such as Dresden Files or Star Wars, should probably be on the fantasy/sci-fi site. A research question about an RPG setting, such as this one, would be a good fit here, unlike any of the other sites mentioned.