Downvoting also allows people to express disagreement without having to give reasons [...] I think this also goes against the purpose of the site.
Maybe not disagreement as such, but it's very often good to express disapproval without detailing the reasons for it. The basic issue here is that a response increases visibility (more, in fact, than an upvote does), and you generally don't want to make things you disapprove of more visible.
The classic example would be deliberate trolling, where a lovingly crafted response detailing everything that's wrong with the post is precisely what you don't want: it wastes your time and encourages the troll. But it's not much different for incoherent crankery or political diatribes or cat pictures: the author might not be encouraged by a response, but you're still wasting other people's time as long as the thread's clogging up Recent Comments.
That said, while I don't feel that downvoting your conversational partners to express disapproval is an abuse of the system in the same way that block downvoting is, I do think it's a bad idea and wouldn't be opposed to a feature limiting it.
I think you're definitely right that we need to be able to control people who stop the site from being an honest exchange of ideas or good-faith discussion. It might be better to have a button to report trolling, flaming or spamming, but not an all-purpose downvote that might be used for other reasons.
The example I think about is a Religious Forum. If they had a "downvoting" feature that was implemented in the same way that the Less Wrong feature is...anyone showing up who asks too many skeptical questions could just be downvoted out of existence...
Last month I saw this post: http://lesswrong.com/lw/kbc/meta_the_decline_of_discussion_now_with_charts/ addressing whether the discussion on LessWrong was in decline. As a relatively new user who had only just started to post comments, my reaction was: “I hope that LessWrong isn’t in decline, because the sequences are amazing, and I really like this community. I should try to write a couple articles myself and post them! Maybe I could do an analysis/summary of certain sequences posts, and discuss how they had helped me to change my mind”. I started working on writing an article.
Then I logged into LessWrong and saw that my Karma value was roughly half of what it had been the day before. Previously I hadn’t really cared much about Karma, aside from whatever micro-utilons of happiness it provided to see that the number slowly grew because people generally liked my comments. Or at least, I thought I didn’t really care, until my lizard brain reflexes reacted to what it perceived as an assault on my person.
Had I posted something terrible and unpopular that had been massively downvoted during the several days since my previous login? No, in fact my ‘past 30 days’ Karma was still positive. Rather, it appeared that everything I had ever posted to LessWrong now had a -1 on it instead of a 0. Of course, my loss probably pales in comparison to that of other, more prolific posters who I have seen report this behavior.
So what controversial subject must I have commented on in order to trigger this assault? Well, let’s see, in the past week I had asked if anyone had any opinions of good software engineer interview questions I could ask a candidate. I posted in http://lesswrong.com/lw/kex/happiness_and_children/ that I was happy to not have children, and finally, here in what appears to me to be by far the most promising candidate:http://lesswrong.com/r/discussion/lw/keu/separating_the_roles_of_theory_and_direct/ I replied to a comment about global warming data, stating that I routinely saw headlines about data supporting global warming.
Here is our scenario: A new user is attempting to participate on a message board that values empiricism and rationality, posted that evidence supports that climate change is real. (Wow, really rocking the boat here!) Then, apparently in an effort to ‘win’ this discussion by silencing opposition, someone went and downvoted every comment this user had ever made on the site. Apparently they would like to see LessWrong be a bastion of empiricism and rationality and [i]climate change denial[/i] instead? And the way to achieve this is not to have a fair and rational discussion of the existing empirical data, but rather to simply Karmassassinate anyone who would oppose them?
Here is my hypothesis: The continuing problem of karma downvote stalkers is contributing to the decline of discussion on the site. I definitely feel much less motivated to try and contribute anything now, and I have been told by multiple other people at LessWrong meetings things such as “I used to post a lot on LessWrong, but then I posted X, and got mass downvoted, so now I only comment on Yvain’s blog”. These anecdotes are, of course, only very weak evidence to support my claim. I wish I could provide more, but I will have to defer to any readers who can supply more.
Perhaps this post will simply trigger more retribution, or maybe it will trigger an outswelling of support, or perhaps just be dismissed by people saying I should’ve posted it to the weekly discussion thread instead. Whatever the outcome, rather than meekly leaving LessWrong and letting my 'stalker' win, I decided to open a discussion about the issue. Thank you!