I've noticed that many new users post questions that fall out of the scope of a particular site and get closed. Often these are very common (such as idea generation on Worldbuilding SE or homework problems on a science SE site). These get closed with only a two-line explanation that doesn't adequately explain what the scope is and why it is needed and why the particular question doesn't lie in scope.
Therefore I think there should be a clear page that explains the concept of scope and also a list of site specific examples of commonly received types of questions from new users that actually fall out of scope. These examples can be crafted using hypothetical users. Examples do help teach scope properly. And it is easier to convince someone that their question is similar to an example that is declared off-scope rather than an abstract explanation like 'idea generation not allowed'.
P.S. This is perhaps even more important than explaining how reputation works, which can be explained to users who actually plan to stay longer than a couple of questions.