The main reason this was closed is because determining what or what does not constitute a hidden feature is highly subjective. Depending on the audience, some features that are considered common knowledge could be considered hidden features. We generally want to focus on the domain of questions that have solid, objective answers, or rather, that solve a clear problem that people have, whereas this question is more focused on trivia.
Furthermore, "List of <X>" questions, as we dub them, generally aren't well-suited for the Stack Exchange format, especially when the question is as popular as this one. Note that navigating the list without OP's quick-link breakdown is a pain because of the way that each individual's answer is separated.