the same way United States Supreme Court Justice Potter Stewart defined porn in Jacobellis v. Ohio, you know it when you see it

Yeah I don't know. It's impossible to draw a clear line, but the overall effect is not hard to identify. If your work supports a community and creates value, you can reasonably expect the community's support. If your work leeches from a community, you can reasonably expect the community to turn on you.

To me slop is identifiable by its effect: it is the type of work which leeches value