This answer of mine: https://stackoverflow.com/a/8845623/103167 used to have a lot of relevant comments from multiple users.
Now they're gone, and not even a note from a diamond mod saying they were deleted.
Any explanation would be nice.
I deleted it. It triggered the "More than 20 comments in three days" flag by Community.
I scanned it, and while there was a good amount of relevant information, the comments section is not the place to put relevant information.
I even gave fair warning that the comment thread was going to be cleaned at 2012-01-13 19:12:12:
Take it to chat, I'm wiping the comments in five minutes.
If there was something really important, then someone should have commented @ me and indicated what was useful/not and users should have pruned the comment list.
Instead, the one comment that came a few minutes after was:
@Ben: Lack of sleep, I thought we were talking about
No indication was given from anyone that this was important.
I deleted them all on 2012-01-13 19:21:31 (over nine minutes later).
That said, comments are not the place to place important, relevant information. If the comment thread is that long, it's a huge red flag possibly indicating some of the following:
Remember, the focus on the Stack Exchange network is the question and answer, not the comments.
We fully support our elected moderators, but realize that all moderators (including and especially myself) are only human and sometimes we make the wrong call -- or we may not even agree amongst ourselves what the "correct" course of action even is. Moderation often requires making tough calls that won't make everyone happy no matter what you choose to do.
We all have the same goal -- to make Stack Overflow the best possible resource it can be, and to keep the noise low and the signal high. We're all on the same team here.
In general, the correct guidance for comment removal is what Robert Harvey already said:
I only delete [whole] comment threads [..] if they have devolved into pointless bickering
However, if there is something of genuine utility in the comments alongside the bickering, then it's less clear what to do. I can recommend:
posting a comment warning the commenters that the comments, while somewhat constructive, have devolved and are at risk of removal.
surgically removing just the bickering comments, if possible, and leave the others that are of genuine utility.
Bear in mind too that sometimes it takes a while to get your feet as a new moderator; I've certainly had many email exchanges from 2009 to now as new moderators were learning the ropes and figuring out the best course of action. We're not perfect, but the important part is that we discuss this rationally here on meta, so that our moderators -- and our community -- can continue to improve.
I wasn't going to post anything, but Xeo's comment:
@Robert: Well... if mods are already allowed to simply nuke the comments, why not preserve them and just remove the noise?
made me change my mind. There's already a link for the users to move the comments into chat. So may I propose that, instead of deleting the comments in toto, the mods first move the conversion into chat, and then delete the comment chain? After deleting the comments, the mods could post a single comment with a link to the chat room to which the comments had just been moved.
That would make everyone happy. Mods could clean up the comment chain, and users could continue their conversation without interruption.
I only delete comment threads in the C++ tag if they have devolved into pointless bickering. However....
The C++ tag is notorious for very lengthy comment chains. Normally I leave these alone, but sometimes these discussions get unnecessarily drawn out over pedantic details. The purpose of comments is to clarify the post, not necessarily to have lengthy discussions about language standards.
While there might be some valuable information in these comment chains, the information that arises in the resulting discussion should be incorporated into the actual post, as the discussion that arises in the comments is, honestly, only of interest to the people having the discussion.
Few future visitors to the post have the patience to sift through a lengthy trail of comments to find whatever nuggets of gold might be there.
I think they weren't deleted solely for the reason given by Robert.
There were two flags on the comment flags, one being an offensive (earlier dismissed by me, as it was confusing as hell) and one being too many comments. The offensive flag referenced a chat transcript, which really didn't help matters much. Mods don't really have the time to sort through tons of BS to sort out a flag. I spent a couple minutes trying to puzzle it out and gave up. I figured if there really was something offensive in there it would get flagged again.
The second flag was a "too many comments" flag automatically set by the system.
What I would suspect (I can't speak for the mod who did the dirty deed) that the two flags in combination led to the judgement that the whole bunch should go down the tubes.
It should be noted that this wouldn't have been a problem if moderators and users had the ability to migrate comments to chat. It's not like this has been asked for on several separate occasions or anything.
Maybe someone should look into giving us the ability to clean up comment threads on our own.