gmail encourages you to archive everything, making it no longer visible. slrn autohides every article as you read it. Both make it easy to rewalk the tree of conversation as new messeages arrive.

Speaking of which, LW used to publish an RSS feed for each "post" or "top-level submission," which makes it easy to use an RSS reader to rewalk the tree of comments under that post in basically the same way.

ADDED. A better "place" for me to have "put" this comment would have been as a reply to this feature request though that feature request and the ensuing discussion suggest that people would prefer to use an interface like lesswrong.com/comments/ instead of an RSS reader to meet the need expressed in the feature request.