An optimization for newspapers to get more online subscribers

Make it so only subscribers can submit comments. If a story has less than say 100 comments, then it can remain free (to get the conversation going). After that, make the user hit a paywall after they’ve written their comment (but before it’s been posted)

Content can be commoditized, but self expression on a well branded page with distribution is not as easily substituted.

You could also enable commenters to pay extra to give their comment better distribution along a sidebar.