Forum rules should be public, and it's generally better for everyone if they're specific, too.

When I report content, it's better when the report form gives me a list of stuff to choose from, and I can check off the box.

When moderators remove content, they should have the same experience. It makes things easier for them.

Users who read the rules don't end up getting upset about wasting time having their content that they worked hard on removed.

I don't think it's a good experience for users if moderators even appear to be power-tripping, and that's often what it looks like if they are moderating without rules.

The exception that I think most people would agree with is that when the rules are specific, you can get trolls who specialize in barely not breaking the rules, while tricking other people into breaking the rules. So, good moderators do need a little discretion, but I think most of their moderation would be easier with specific rules.