Or maybe it's a setting on each category as to whether to show it if there's no permission. It's default setting would be calculated statistically just how allow-ratings is.

What I said to a customer, which is an extension of this for Download Categories (let's you fully browse the category, just not download):

You'd be able to browse through the Download Category before purchasing full access, but any attempt to download a Download would take you to a purchase screen). There'd also be a message over the top explaining this, with a link. This will allow a customer to see exactly what they'd be purchasing. Time: 2 hours.

This is now implemented for 10.1 for downloads as a global option, and actually I don't think it makes sense to do it for any other content types. Downloads are the only one where the main goods are separated from browsing.