My personal downvoting policy is this: I tend to refrain from downvoting unless the comment really is atrocious in a way that I feel ought to be intuitively obvious. This has very little to do with the actual content of the comment and a great deal to do with the presentation; for example, I won't downvote a comment that I disagree with, but I will downvote a comment that I perceive as adding nothing to the discussion.
In practice, this means that I do not downvote very often; in the totality of my time here on this site, I believe I've downvoted less than ten comments. The less atrocious comments, however, I neither upvote nor downvote; instead, I write a reply discussing what I disagree with in the comment and what can be done better. (Note that in this case I actually upvoted your comment for being well-written and thoughtful, even though I disagree with your conclusion.) In that sense, my downvoting rationale adheres to your "exclusivity" principle. And yet I feel that downvoting based on pure disagreement, which you appear to be endorsing, is rude and is not a healthy behavior for the community, seeing as it basically serves to provide negative reinforcement every time someone says something contrarian, which is hardly conducive to an atmosphere of cooperative discourse. Comments that you disagree with should be replied to, not downvoted without explanation, unless there's some other factor unrelated to content that caused the downvote.
Most of the time, however, when I'm downvoted, I find that I can hardly discern the reason. I strive to make clear comments that express my point concisely without going on for to long, so whatever the reason is, it can't be presentation. Content, then? But if I'm saying something stupid or wrong, I'd very much like to know! Naturally I don't think I am, so if someone randomly downvotes one of my comments without telling me why, I'm forced to either (a) make unsupported speculations about what I'm doing wrong, or (b) disregard the downvote as uninformative and therefore not worth thinking about. Neither option seems particularly appealing to me, so I'd say that as a strategy to positively impact the behavior the of person being downvoted, your suggested policy of downvoting fails. Is there some other reason you're endorsing it?
I did not suggest downvoting purely on account of disagreement. It is true that if I had not responded to Gondolinian's comment, I might have downvoted it. But not just because I disagree, but because I find complaints about downvoting unpleasant and would rather see less of them on the site.
In general there might be many other reasons for downvoting which do not necessarily involve disagreement, such as vagueness, excessive verbosity, illogical reasoning, and so on. Again, of course you can simply respond and mention those things, but again in that case t...
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