Strategies that require tracking the user have a fatal flaw. For public articles and public comments, the user can be logged out while reading and only log in to comment. This happens to be what I have done just now for this post.
Additionally, the user might not even be on the site at all while they spend 10 minutes carefully reading the article, or dozens/hundreds on the site. I almost exclusively read web sites via rss, but your scheme doesn’t know that. Consider also instapaper.
In these cases, it’d appear that I arrived fresh to the discussion and started posting, but that’s not true.