If people had to actually write a comment to voice their disagreement, everyone would benefit.
No, if someone e.g starts randomly insulting people, or being utterly clumsy in their reasoning, and then everyone would have to voice their disagreement, repeatedly, that would be noise atop of noise. It would crowd out any signal in the discussion.
Some types of disagreement at least need be able to be expressed without noising up the place. One of these is "You're very obviously wrong, and this comment is bad for this place, and it's bad for this place that I even have to use two sentences to you to explain why".
The person who is wrong would benefit by being provided an actual explanation for why someone disagrees
Even in the better cases scenario where there are no trolls or spammers involved -- what makes you think that you have to choose between only the options "Someone disagrees with you and you don't know why" and "Someone disagrees with you, and you're told why" -- and you don't consider the third option "You have no way of knowing if anyone agrees or disagrees with you"?
You seem to think that the only alternative of "downvoting and not commenting" is "commenting" -- so that we need only prevent downvotes to encourage discussion. Ofcourse there are atleast two alternatives instead, and one of them is "not downvoting and not commenting either".
Then the members wouldn't get any feedback, not even that of a downvote/upvote; they wouldn't even know if anyone even read their posts.
I'd find that even more depressing. An upvote or a downvote is atleast knowledge that someone read what I had to say.
The person who disagrees would have to be more specific and maybe name some concrete reasons for their disagreement and that way notice that
No, another option they would have is to not reply at all. Or to call other people names. Or to lower the tone of the conversation.
A downvote is an easy way to tell someone "I believe you're doing something wrong" without the addition of noise.
...and you don't consider the third option "You have no way of knowing if anyone agrees or disagrees with you"?
Right, what about an agree/disagree feature?
People who go back and downvote every post or comment a Less Wrong user has ever made, please, stop doing that. It's a clever way to pull information cascades in your direction but it is clearly an abuse of the content filtering system. It's also highly dishonorable. If you truly must use such tactics then downvoting a few of your enemy's top level posts is much less evil; your enemy loses the karma and takes the hint without your severely biasing the public perception of Less Wrong's discourse.
(I just lost over 200 karma in a few minutes and that'll probably continue for awhile. This happens to me every few weeks. Edit: I mean it's been happening every few weeks for a few months for a total of only three or four. Between 400 and 700 karma lost total I think? I don't mean to overstate the problem.)