I use the Hacker News Enhancement Suite extension for Chrome (by HN user etcet).
https://chrome.google.com/webstore/detail/hacker-news-enhanc...
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:
http://userscripts.org/scripts/show/110317
I use the Hacker News Enhancement Suite extension for Chrome (by HN user etcet).
https://chrome.google.com/webstore/detail/hacker-news-enhanc...
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.