You are aware, Mariano, that anyone can look at your profile and see that you have cast a miniscule number of votes in proportion to your participation? I have a hard time believing you are being serious. I thought not voting much was a matter of principle for you until now...

The only sane etiquette I see as worth developing in the site is to vote, to encourage people to actually vote up and down

I agree. The MO community is actually quite good about voting freely. According to the SE "top sites" leatherboard, MO posts get an average of 3.8 upvotes in the first 24 hours they're up.

I've gotta say: I think this policy is way too interventionist. I can understand isolated conversations that really get off the rails, and which is of general benefit to delete. But I don't think it's a good idea, or a worthwhile use of moderator time, to "clean up" a discussion like this when nothing untoward happened.

I agree that just deleting the comments without a trace was pretty creepy, but I can sympathize with the desire to "clean up" a comment thread. It's annoying to have to wade through a list of irrelevant comments and fish out the ones that are worthwhile, and I don't think it's a waste of time to do something about it. After all, it's not a waste of time to vote answers up and down, the main purpose of which is to sort out the best answers so that people can get to the good stuff. Similarly, it's not a waste of time to vote up comments, which visually distinguishes them so that it's easier for people to find them later (indeed, in long comment threads, only the five most highly voted comments appear unless you ask for more). We don't have a way to downvote comments, but that's what Scott really wanted to do: mark those comments so that they would be easier to ignore somehow.

I think deleting somebody else's comment is far too likely to come across as a violation, so we shouldn't do it. Given that deleted comments are completely invisible even to the comment owner and to moderators, I think moderators should shoulder the overhead of sending an email to (or leaving a comment for) the comment owner to confirm before deleting a comment.

In the case of the quantum groups books question, the comments were pretty meta. What do people think about the following approach? If Scott (or another moderator) really thinks the comments interfere with the actual math going on in the thread, he pastes them into a new thread here on meta, deletes them, and leaves a comment to the effect of "I've moved the first four comments to http://tea.mathoverflow.net/discussions/1234". My intuition is that most people would be okay with that, but if not, please speak up.

Oh, and re: http://mathoverflow.net/questions/16977/why-do-my-quantum-group-books-avoid-homotopical-language, I honestly don't remember which comments I deleted! I think it was actually along exactly the same lines -- someone complaining about a mysterious downvote, and at the time I arrived the vote total was healthily positive. If the protagonists care, we can reconstruct the comments using our new database dump superpowers!

As a demonstration of those superpowers (I finally got ahold of the dump I asked for), Scott deleted the first four comments on that thread. They were

Why was this question downvoted? It seems a valid question to me. <UserId>394</UserId>
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I can't imagine why anybody would say that this question looks like homework. <UserId>135</UserId>
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The only reason I can imagine for a downvote --- I voted +1 --- is that it takes a while to get to a question (and the title isn't posed as a question). <UserId>78</UserId>
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Is the only reasonable reason for a downvote that the question be homework? <UserId>1409</UserId>

This is, I understand, implemented on StackOverflow -- at least for users above some reputation threshold (I suspect the threshold is a hack to limit server load, as this operation isn't database friendly for historical reasons). Quite possibly, after we come out of beta, the StackExchange folk will be migrating features over from StackOverflow. We may just gain this as part of this migration, we may have to go ask for it, I'm not sure.

Actually I agree on this point: while downvoting without explaining is definitely a problem (perhaps not generally, but often in specific cases), the overall low rate of voting is something that I'd be much happier about improving.

Regarding the comments above the Anton could salvage (fpqc and jose): By the time I got the the question, the total vote count was at +6 (where it stands right now, I think). As such, I thought the comment "why is this being downvoted?" was no longer relevant. I stopped to check for systematic downvoting against fpqc, which has been a problem in the past, found nothing and decided to delete both comments. Deleting fpqc's was fine: at some point I received explicit permission from him to delete his comments when I felt it appropriate, and that hasn't been retracted. Deleting Jose's was, I agree, especially seeing the kerfuffle in this thread, problematic.

Personally, I would prefer an explicit policy that questions which concern the operation of mathoverflow, rather then mathematics, may be deleted by moderators when they feel that the comments are no longer relevant, or indeed otherwise at their discretion! I acted today on the mistaken presumption that this was okay.

Now, going forward, I'll be much more careful to leave my own comment, to the effect that I've deleted part of a comment thread, for some reason. I've done this several times in the past when comment threads have got out of hand, and I think this has been well-received, or at least acceptable. Further, I'll be more hesitant to delete others comments at all. Earlier in the life of mathoverflow, quite a few times I'd emailed the authors of a batch of comments I thought were either inappropriate or no longer relevant, and proposed that I delete all of them. To my memory, this was always agreed on. More recently, I guess I've had slightly less enthusiasm for the overhead incurred, and just been bold.

Leaving aside that fact that I didn't explain what I was doing, which I absolutely agree would have been better, I think one important difference we might be having here is our intents. The authors of the comments are writing them in order to have a conversation. I've been deleting them in order to optimise the long-term value of the page to other readers. There's some balance to be struck here, and I'm keen to hear where people think it should lie.

I really do not see why it is sensible to vote up without explaining oneself and not to vote down without explaining oneself...

The only sane etiquette I see as worth developing in the site is to vote, to encourage people to actually vote up and down (There are question with more than 1 kiloviews and 9 votes!---of course, I cannot tell whether 9=509-500 or 9-0, it'd be useful to have questions and answers also display the total number of votes cast, for otherwise the numbers are essentially meaningless)

Thanks for the -1, guy. – fpqc 2 hours ago
Why is this question being downvoted? That's the second down vote today that I don't understand. – José Figueroa-O'Farrill 2 hours ago

I did not agree to anything...

Sorry, then I misunderstood what happened.

These mysteriously vanishing comments are very strange. I'll try to track them down and see what's going on.

Disagreement is part of mathematics, as far as it is a human activity. For example, the debate on the merits of Bourbaki is not the exclusive realm of overzealous undergradates in the search of more useful causes, and it has involved great titans---sometimes in much harsher terms than what we saw here, and in much more public and holy places (MR!). Keeping the site focused on its purpose, which is not to be a discussion site, is extremely good, but if it is to play even small role as the public face of mathematics, then I see no reason to hide the lively disagreements we do have :D

Strangely, instead of letting the mini-thread die off by simply ignoring it, the whole set of comments vanished. I asked what had happened, and Andy Putman explained.

Is there not a non-destructive way to deal with this type of situation? Does the software not allow, say, locking comments? Was the simple-minded approach of clearly stating in a comment that the off-topic discussion was out of place? (I don't know: maybe someone did say that, but I did not see it before the whole thing disappeared...)

It did seem to me rather, hmm, abrupt...