one knows it when one sees it

delegate this question to the janitors

The problem in this (and that) approach -- which is what we've always done -- is that, when someone objects to a certain action (e.g. "My post wasn't off topic!"), we don't have something we can point them to. People are not satisfied with "sorry, it's janitor's discretion". They have to live with it, yes; but it's not satisfying, and does nothing to abate the flamefests.

I'm not suggesting that the definitions should be very specific and rigid. In fact, with my Process Improvement hat on, I'd say probably the best way to start would be for the janitors to write down what criteria, heuristics, etc. they use, no matter how nebulous. Someone could then take that material and try to distill it for the documentation.