In case you don’t read release notes(I myself usually don’t…), GreatNews build 354 added ability to track comments automatically. If an article has comment feed, GreatNews will show “Track Comments” link like this:
If you click the link, GreatNews will start monitoring the article, and mark it as unread when new comments posted.
This function is enabled for new users by default. If you upgraded from previous version, you can enable it in Tools->Options->Features.