If upvotes/downvotes are public, some people are going to reward/punish those who upvoted/downvoted them.
It can happen without full awareness… the user will simply notice that X upvotes them often and Y downvotes them often… they will start liking X and disliking Y… they will start getting pleasant feelings when looking at comments written by X (“my friend is writing here, I feel good”) and unpleasant feelings when looking at comments written by Y (“oh no, my nemesis again”)… and that will be reflected by how they vote.
And this is the charitable explanation. Some people will do this with full awareness, happy that they provide incentives for others to upvote them, and deterrence to those who downvote. -- Humans are like this.
Even if the behavior described above would not happen, people would still instinctively expect it to happen, so it would still have a chilling effect. -- On the other hand, some people might enjoy to publicly downvote e.g. Eliezer, to get contratian points. Either way, different forms of signalling would get involved.
From the view of game theory, if some people would have a reputation to be magnanimous about downvotes, and other people would be suspected of being vengeful about downvotes, people would be more willing to downvote the former, which creates incentives for passively aggressive behavior. (I am talking about a situation where everyone suspects that X downvotes those who downvoted him, but X can plausibly deny doing that, claiming he genuinely disliked all the stuff he downvoted, and you can either have an infinite debate about it with X acting outraged about unfair accusations, or just let it slide but still everyone knows that downvoting X is bad for their own karma.)
tl;dr—the same reasons why the elections are secret
EDIT:
After reading Raemon’s comment I am less sure about what I wrote here. I still believe that public upvotes and downvotes can cause unnecessary drama, but maybe that would still be an improvement over the situation when a reasonable comment gets 10 downvotes from sockpuppet accounts, or someone gets one downvote for each comment including those written years ago, and it is not clearly visible what exactly is happening unless moderators get involved (and sometimes not even then).
On the other hand, I believe that some content (too stupid, or aggressive) should be removed from the debate. Maybe not deleted completely, but at least hidden by default (such as currently the comments with karma −5 or less). But I agree that this should not apply to not-completely-insane comments posted by newbies in good faith. Such comments should be merely sorted to the bottom of the page. What should be removed is violations of community norms, and “spamming” (i.e. trying to win a debate by quantity of comments that don’t bring new points, merely inflate the visibility of the already expressed ones).
At this moment I am imagining some kind of hybrid system, where upvotes (either private or public, no clear opinion on this yet) would be given freely, but downvotes could only be given for specific reasons (they would be equivalent to flagging) and in case of abuse the user could lose the ability to downvote (i.e. the downvotes would be either public, or at least visible to moderators).
And here is a quick fix idea: as the first step, make downvotes public for moderators. That would at least allow them to quickly detect and remove Eugine’s sockpuppets. -- For example, moderator could have a new button below each comment, which would display the list of downvoters (with hyperlinks to their user pages). Also, make a script that reverts all votes given by a user, and make it easily accessible from the “banned users” admin page (i.e. it can only be applied to already banned users). To help other moderators spot possible abuse, the name of the moderator who started the script for a user could be displayed on the same admin page. (For extra precaution, the “revert all votes” button could be made inaccessible for the moderator who banned the user, so at least two moderators must participate at a vote purge.)
If upvotes/downvotes are public, some people are going to reward/punish those who upvoted/downvoted them.
It can happen without full awareness… the user will simply notice that X upvotes them often and Y downvotes them often… they will start liking X and disliking Y… they will start getting pleasant feelings when looking at comments written by X (“my friend is writing here, I feel good”) and unpleasant feelings when looking at comments written by Y (“oh no, my nemesis again”)… and that will be reflected by how they vote.
And this is the charitable explanation. Some people will do this with full awareness, happy that they provide incentives for others to upvote them, and deterrence to those who downvote. -- Humans are like this.
Even if the behavior described above would not happen, people would still instinctively expect it to happen, so it would still have a chilling effect. -- On the other hand, some people might enjoy to publicly downvote e.g. Eliezer, to get contratian points. Either way, different forms of signalling would get involved.
From the view of game theory, if some people would have a reputation to be magnanimous about downvotes, and other people would be suspected of being vengeful about downvotes, people would be more willing to downvote the former, which creates incentives for passively aggressive behavior. (I am talking about a situation where everyone suspects that X downvotes those who downvoted him, but X can plausibly deny doing that, claiming he genuinely disliked all the stuff he downvoted, and you can either have an infinite debate about it with X acting outraged about unfair accusations, or just let it slide but still everyone knows that downvoting X is bad for their own karma.)
tl;dr—the same reasons why the elections are secret
EDIT:
After reading Raemon’s comment I am less sure about what I wrote here. I still believe that public upvotes and downvotes can cause unnecessary drama, but maybe that would still be an improvement over the situation when a reasonable comment gets 10 downvotes from sockpuppet accounts, or someone gets one downvote for each comment including those written years ago, and it is not clearly visible what exactly is happening unless moderators get involved (and sometimes not even then).
On the other hand, I believe that some content (too stupid, or aggressive) should be removed from the debate. Maybe not deleted completely, but at least hidden by default (such as currently the comments with karma −5 or less). But I agree that this should not apply to not-completely-insane comments posted by newbies in good faith. Such comments should be merely sorted to the bottom of the page. What should be removed is violations of community norms, and “spamming” (i.e. trying to win a debate by quantity of comments that don’t bring new points, merely inflate the visibility of the already expressed ones).
At this moment I am imagining some kind of hybrid system, where upvotes (either private or public, no clear opinion on this yet) would be given freely, but downvotes could only be given for specific reasons (they would be equivalent to flagging) and in case of abuse the user could lose the ability to downvote (i.e. the downvotes would be either public, or at least visible to moderators).
And here is a quick fix idea: as the first step, make downvotes public for moderators. That would at least allow them to quickly detect and remove Eugine’s sockpuppets. -- For example, moderator could have a new button below each comment, which would display the list of downvoters (with hyperlinks to their user pages). Also, make a script that reverts all votes given by a user, and make it easily accessible from the “banned users” admin page (i.e. it can only be applied to already banned users). To help other moderators spot possible abuse, the name of the moderator who started the script for a user could be displayed on the same admin page. (For extra precaution, the “revert all votes” button could be made inaccessible for the moderator who banned the user, so at least two moderators must participate at a vote purge.)