Many comments become noise over time. At the same time, some comments add valuable information to the question or answer and should be preserved permanently.
So far, all efforts to conceive a clean-up feature that makes it easy to distinguish worthy comments from noise have failed. When seeing a comment thread that should be deleted, the current modus operandi is to flag for moderator attention. That is arguably not the ideal way to go about it in the long term. It is work intensive and moderators will often be unable to judge the value of a domain-specific comment.
So how about letting comments decay by default, with the option of making selected ones permanent? This is not my idea; it has been thrown around repeatedly, for example in this conversation between Shog and Rich B.
Here's a concrete feature request.
By default, give comments a limited lifetime, for example 45 days. A comment that hits the maximum age mark vanishes from the thread.
Moderators and 10k+ users can see these vanished comments (but not actively deleted ones) using a new link:
To the right of every comment, add a new link that shows on hover:
Clicking on that link makes the comment permanent, ie. it does not vanish when reaching the maximum lifetime. It works like all comments do currently. Other than that, it remains a perfectly normal comment. It can still be deleted by the author or by moderators, of course.
When the author of the comment clicks on that link, the comment becomes permanent immediately. For other users, "Make permanent" could be a 2k+ feature or a vote that needs two votes (not sure about that part - would have to be throught through separately)
Permanent comments could be highlighted slightly.
Like the number of total comments, the number of permanent comments per day could be limited to prevent abuse of the "make permanent" feature, especially in heated discussions.
We would finally have ORDER!!!!!!!!!!
Discussions, requests for clarification etc. would clean themselves up automatically after four to six weeks. Only the stuff that someone wants to actively preserve remains.
Cleaning up comments becomes less important. Mods have less work.
The change would create added incentive to incorporate important information into questions and answers.
Occasionally, important information will vanish because users don't understand the system and no other users turn up to mark it permanent. With a system that decays comments by default, there is no way to completely prevent this. I feel it would still be worth it, as comments already are second-class citizens and fair game for mod deletion.
The feature could be abused by those who think everything they say deserves to be made permanent, or simply don't understand how the system works. That is not much of a problem though: their comments will simply work like every comment does right now, and can be easily cleaned up by moderators.
As the feature is not completely easy to discover, and differs from the old behaviour, creating permanent comments would become a privilege reserved to those who know how to use the sites. That is arguably a bit unfair, but could be remedied e.g. by showing a message before the user posts their first comment under the new rule.
To avoid bloodshed, when introducing this feature, make all existing comments permanent; apply the new rule to new comments only. Show banners and warning messages to inform users about the new behaviour.