The current modus operandi to clean up comment threads that are extremely chatty, obsolete, or consist of pointless discussion is to flag them for moderator attention.
This works; however, it would of course be way cooler if the community could clean up comment threads on its own. No suggestion to that end has proven viable so far, though.
Here's one more:
Add a new comment flag, "obsolete in the long run." placement and wording are open to discussion:
There would have to be a rep threshold for those kinds of flags. This threshold would have to be determined separately.
Once two users have cast such a flag, the comment will get a small star:
When I hover over the star, I will see when the comment is scheduled to expire.
Maybe when one flag has been cast and one more user's flag would be necessary to mark the comment, half a star could be shown to signal to other users that the comment is "waiting" for a second flag.
The comment will be removed 60 days after the last flag was cast.
Comments get cleaned up, but not while the discussion is still ongoing. Friendly "thanks, that worked for me!" comments can stick around for a while, and reach the addressee. "Social" conversations can exist for a while, but those of us who think that's noise can rest assured they will be gone in two months, when they no longer matter.
Two dedicated users can clean up an entire comment thread without need for a mod.
The task of cleaning up comments would be in the hands of users who are familiar with the question's tags and can better identify obsolete comments than mods can.
The upcoming deletion of a comment is visible to anyone familiar with the system, and 60 days is ample time to see and, if necessary, save a comment marked this way. A "unflag" feature might be an option - or, more simply, if you see important information in a comment with a star, just re-post that comment.
- Two angry/troll users can collaborate and remove content from the site. This might be a problem, although I am inclined to think it's more of a theoretical nature. The current "flag as offensive" voting threshold (I think it's three votes?) already almost allows this, and I'm under the impression rampages are usually the work of a lone gunman.
Despite my long list of related suggestions, I don't think comment noise is a huge problem on Stack Overflow. (I really don't, and this will be my last feature request in this direction. :) The way things currently work (alert a mod and ask them to clean up) seems viable. Also, threads with more than 5 comments will get auto-collapsed by default, and only upvoted comments will be displayed. So, "we don't need community-run comment cleanup" is a valid answer here.
But it still would be cool to have a community-driven comment cleanup process IMO, and it might also reduce mod workload, which is always a good thing.