I hopped over to meta to suggest the ability to downvote comments, and it appears that it had been brought up several years ago as Allow downvoting comments, but the powers that be decided to decline it.
I don't agree that including downvotes for the reasons the asker stated is good, but most of the rest of the discussion shows a pretty heavy consensus in favor of the ability to downvote comments, for reasons other than what the asker posted. Such reasons included reducing the signal-to-noise ratio for moderator flags (as many comments are flagged likely because there's not option to simply downvote them, and the system itself discourages getting "chatty") as well as in the discussion itself, and creating a decay mechanisms for comments that are simply just wrong, but don't otherwise qualify for flagging. It was even noted that Jeff, himself, said that upvoting was only half of the equation and that most comment flags aren't justified.
Why, then, was the decision made to reject this feature request? Given the age of that question, should the feature be revisted?
Edit: Since this question spawned more discussion around this idea as a feature request, instead of what I was originally intending, here are the reasons I think it would be a good idea to implement:
- It reduces noise for the moderators. One of the things mentioned by both a user and Jeff Atwood himself was that a large number of flags on comments are not justified. This says to me that people are flagging comments in lieu of a better mechanism for dealing with a given comment. Provide something between "doing nothing" and "nuking a fly," and you reduce the unnecessary work the moderators have to do, and allow them to focus on the real issues that have been flagged.
- It reduces visual clutter for users. Shog9 linked this post in one of his comments. It's the blog entry for the release of the "Top N Comments" mechanism. It also changed how the voting an flagging icons were displayed (ie - they required mouseover to display the icons). The reason? To reduce visual clutter. We can even take this a step further, and visually hide comments that go negative a certain amount, the same way the system currently hides non-voted comments when certain thresholds are reached, or the same way other systems that use voting in comments hide downvoted comments, regardless of how many total comments exist (CSS-Tricks, Slashdot, etc).
- Upvoting conveys only half of the information, and upvoting alone doesn't provide enough information. Within the posts that were linked in the comments section, there was a link to this post by Jeff criticizing Hacker News for their lack of a downvote. In it, he states that only having upvotes only conveys half of the information. Does 0 mean it's bad? Or does it just mean it's boring enough to not earn upvotes? If there's no downvote system, we don't know, because we're missing the other half of the equation - the people that don't agree with comment.
- It balances out bad comments that were upvoted for whatever reason. A comment that doesn't give good information, but was upvoted -- even once, and for any reason (such as a witty remark in another part of the comment) -- will be displayed in favor (to the system) over a comment that corrects the bad information that doesn't get any upvotes (the most common reason I've seen for this is that the comment was added after the initial flood of viewers has died out) when the comment threshold to trigger Top N is reached. With a downvote system in place, the person correcting the comment can downvote the comment in question, thus potentially pulling it out of the Top N list and giving it a similar visual weight as their own comment, making the correction less likely to be missed when the misleading one is seen (because the mind was drawn only to the "highlighted" comments).
- It doesn't change the existing psychological meaning of comment voting. One of the common things I've seen so far is that upvotes in comments are for agreement, without the need to post superfluous comments saying things like "I agree!". That's great! And in my opinion, all the more reason to have downvotes. The metaphore then becomes one of downvoting one comment (to show disagreement), and upvoting an existing comment that disputes the former one. Why not just upvote the latter and be done with it? Because not having the downvote inflates the value of the misleading comment, when it should be devalued. Without a downvote mechanism, you have to upvote all of the other comments that you deem "better than" the one you want to downvote in order to devalue the misleading one, and by doing so, you're showing agreement to those, even if they otherwise wouldn't be good enough to warrant an upvote on their own (ie - better than the misleading one, but not great or all that worthwhile). This is an absurd measure, and thus, not done. As a result, comments that one disagrees with maintain the illusion that more people agree with it than is reality.
- It brings comment voting in line with voting on the meta sites. One could also ask - if upvote only is sufficient for comments, why isn't sufficient for the meta sites, which use the same agreement/disagreement metaphor? Any negative value question or answer on the meta sites will give you that answer, and it's still essentially the same as my point #3 - only allowing upvotes (and thus, not allowing a score go below 0) only provides half of the information. If the score on this question was 0. Does that mean no one agrees? Or does that mean no one agreed enough to vote for it? Judging by only the responses as of this writing, there's one person that actively disagrees with posted question, one person that actively agrees, and two people that are neutral on the topic. That's a far cry different from the 5 upvotes/agrees and 9 downvotes/disagrees this post has gotten (even if you put the two neutral responses on one side or the other).
- It creates UI symmetry and consistency. One of the answers to the old question mentions this point. It may or may not be a small thing, but including the ability to downvote comments creates consistency with everything else with a voting mechanism on the site, reducing user confusion.
- Comments are no longer second-class on SE. The developers saw to that when they implemented the Top N Comments function. This gives the number next to a comment inherent value, because the system itself filters on it. They also elevated the value of comments and discussion when they implemented notifications for the "@user" mechanism, as well as when they changed Top N Comments to no longer sort by number of votes (a change made shortly after the initial launch of Top N Comments).