So I figured out what makes a good question: what is likely to generate good answers. What is a good answer? I'm sure this has come up before, but, what, basically, is a good answer, beyond one that refers to philosophical literature to back up its claims?
I mean you're kind of right that good questions and good answers go together, but if you want to say that good questions are good because of their answers, then that's problematic. It doesn't give space to poor questions with answers better than they deserve (super lazy questions, very basic questions which could've been easily answered had the OP read a page or two in a book or Wikipedia, questions which were closed because they're entirely opinion based but which received a brilliant answer expressing one side of the debate). It would mean that all old unanswered questions are bad, rather than just obscure or difficult to find resources for answering.