If I think anti-feedback rules are harmful (and I do), I am downvoting this post no matter what (and I did).

Unfortunately, just the presence of the rule or even implication that this is a rule is sufficient to counteract a lot of such feedback. It's not enough for some people to ignore the anti-feedback convention and vote anyway, it might be necessary to take the anti-feedback convention down explicitly as well, to indicate clearly that it doesn't apply to the post.

(Notice how the post managed to stay at exactly 0 Karma this whole time, with only temporary fluctuations.)

Edit: It seems that I was wrong, after several hours the post started going into the negative, and is currently at -10, despite no official change of rules; dissenting feedback was strong enough. This thread might have played a role.

that makes it more difficult to vote such posts out of the forum than similar posts without the rule, thus making expected quality introduced by these posts lower than average.

That's the whole point. The objective of these threads is not to "introduce quality" (and Karma does not measure quality, but popularity, which unfortunately isn't always the same thing); it's simply to have fun. Lots of people are turned off this blog because they find us stuffy, self-important, and unwelcoming. Having threads that encourage another kind of attitude is, I feel, downright healthy.

In fact, I have half a mind to change the next experiment from "don't downvote or upvote" to "please downvote or upvote according to niceness and civility, not "quality" or "intelligence". I want there to be spaces in LW where people aren't being snippy at each other, and where the response to a post you don't approve of but can't be bothered to actually argue against is to down-vote it.

There is only one circumstance in which I for one ever consider downvoting someone, and it's if I feel they're being unreasonable on purpose; wilfully ignorant, or deliberately and smugly obnoxious. I don't down-vote on mistakes, beginner's ignorance, or stupidity.

Edit: I hereby declare this subthread a special meta-discussion exception to which the anti-feedback rules don't apply :-)

Please don't do that retroactively; you're violating tacit consent. If you'd done that from the start, I wouldn't have replied to you in the first place, because I you and I have very different criteria for what merits an upvote or a downvote, and any interaction between you and me will turn to my disadvantage.

In fact, I find the entirety of the karma system insanely dangerous, and would merit a discussion thread of its own. Supposing your attitude annoyed me sufficiently, what's to stop me from searching your post history and downvoting every single post you ever wrote under this handle at LW? I would never do that, because I think that would be evil and I don't like to think of myself as evil,. but you only have my word for it. In fact, what's to stop you from downvoting every single post I make, simply on the grounds that you don't want someone like me on this blog?