Suppose some comment thread has a thousand comments. Without a karma system (as e.g. on Slate Star Codex), I can either waste several hours reading them all, or quickly scroll the page looking for something catching my eye and hope I don't miss something interesting. With a karma system, I can entrust the readership with the task of telling me which comments are the most worth reading, and read them first.
Entrusting the readership to tell you what's good is what you want the karma system to do, but it's not what the karma system is actually doing. Aside from sockpuppets, tribal voting, etc., it's just not possible to generate good recommendation systems from karma systems (otherwise everyone would be doing it). Even more sophisticated "recursive" systems like pagerank don't really work due to collusion, link farms (sockpuppets, basically), and related issues. Google moved away to a more complex system and has a full time police force to try to ...
I've gotten sufficient evidence from support that voiceofra has been doing retributive downvoting. I've banned them without prior notice because I'm not giving them more chances to downvote.
I'm thinking of something like not letting anyone give more than 5 downvotes/week for content which is more than a month old. The numbers and the time period are tentative-- this isn't my ideal rule. This is probably technically possible. However, my impression is that highly specific rules like that are an invitation to gaming the rules.
I would rather just make spiteful down-voting impossible (or maybe make it expensive) rather than trying to find out who's doing it. Admittedly, putting up barriers to downvoting for past comments doesn't solve the problem of people who down-vote everything, but at least people who downvote current material are easier to notice.
Any thoughts about technical solutions to excessive down-voting of past material?