So you want to reread an interesting comments section at point t1 in time (when there are n1 comments) but already read the whole section at point t0 in time (containing n0 comments), t0 < t1, n0 < n1.<p>... obviously you don't want to reread ALL the old comments?
I use the Hacker News Enhancement Suite extension for Chrome (by HN user etcet).<p><a href="https://chrome.google.com/webstore/detail/hacker-news-enhancement-s/bappiabcodbpphnojdiaddhnilfnjmpm" rel="nofollow">https://chrome.google.com/webstore/detail/hacker-news-enhanc...</a><p>When you revisit a comment thread it marks which comments are new since your last visit. You can see an example in the second screenshot in the Chrome Web Store.
I'm not 100% I understand but I find that the greasemonkey script "Hacker News Threadify" makes reading and re-reading comments much better:<p><a href="http://userscripts.org/scripts/show/110317" rel="nofollow">http://userscripts.org/scripts/show/110317</a>