the ad-hoc mob rule that bothers me
(shrug) One person's "ad-hoc mob rule" is another's "collective self-moderation".
For my own part, I endorse the collectively self-moderating aspect of LW, of which downvotes are an important aspect. Yes, it makes the community vulnerable to various forms of self-abuse. Eliminating it also makes the community vulnerable to various forms of self-abuse, which are not clearly superior, to say the least.
The idea that one could be severely punished for an action that didn't violate any specific rule, but was merely something many in the community disagreed with, would be very chilling.
For my own part: I endorse people downvoting what they want to see less of on the site.
If Sam wants to see less of George posting on the site, it follows that I endorse Sam block-downvoting every one of George's comments. I'm a little squeamish about that, and I would prefer that Sam had different preferences, but if it comes down to that I stand by the endorsement.
If I post something that many in the community disagree with, and those community members want to see less stuff they disagree with, I endorse those community members downvoting me. That I didn't violate any specific rule is, to my mind, entirely irrelevant; I would prefer that our goal not be to encourage rule-compliance.
I do recognize that many people here use different downvoting metrics than that... e.g., downvote-what-I-disagree-with, downvote-what-I-oppose-socially, downvote-what-I-consider-overly-upvoted, downvote-things-that-evoke-negative-emotional-responses, various others. I don't endorse those metrics, and I'd prefer they didn't do that, but I acknowledge that interpreting karma behavior correctly requires recognizing that these people exist and do what they do.
Even leaving all of that aside, I also recognize that many people here have different preferences than I do regarding what kinds of things get said here, and consequently things get downvoted that I upvote, and things get upvoted that I downvote. This is as it should be, given things as they are.
I think you misunderstand. I approve of downvoting (and disapprove of certain ways of using it), but I disagree in the strongest possible terms with Dentin's "I'd like to see a search done for the culprit, have them publicly exposed, and their account permanently locked or destroyed."
I'm just tired of the signal pollution, and would like to be able to use karma to honestly appraise the worth of my articles and posts, without seeing 80% of my downvotes come in chunks that correspond precisely to how many posts I've made since the last massive downvote spree.
EDIT to add data points:
Spurious downvoting stopped soon after I named a particular individual (not ALL downvoting stopped, but the downvotes I got all seemed on-the-level.)
One block of potentially spurious downvoting occurred approximately one week ago, but then karma patterns returned to expected levels. I consider this block dubious, because it reasonably matches what I'd expect to see if someone noticed several of my posts together and disagreed with all of them, and did not match the usual pattern of starting with the earliest or latest post that I had made and downvoting everything (it downvoted all posts in a few threads, but not in other threads), so I'm just adding for completeness.
Spurious, indiscriminate downvoting started up again approximately half an hour ago on Sunday (12/1/2013), around noon MDT.
Edit: And now on Tuesday, 12/3/2013, at 10 AM, I'm watching my karma go down again... about 30 points so far.
Edit: And now on Saturday, 12/14/2013, at 2 PM, I'm watching my karma go down again... about 15 points so far, at a rate of about 1-2 points per second.