I understand the abundance of downvote-related questions that are asked here, but I didn't find any related to what I propose, so here is my thought:
I wholeheartedly aggree that the reasons for one's downvote are personal, subjective, and should not be "rulefied". This is not the issue.
What happens is the following: If you post an answer, you almost surely know why you think it is a good one. Therefore, an upvote is a "natural" follow-up for that: it needs no justification. A downvote is something that you do not expect for (in theory) and, with great chance, you don't know why happened: but it can be useful. It serves to alert and to motivate the person to modify their answer properly: I think this may belong among the most prominent objectives of a downvote (along with alerting the rest of the community that maybe this answer is not worth reading etc.).
So, why withhold this potential benefit of the downvote? Most of times, what happens in a downvote is the following:
Userthatwasdownvoted: Why the downvote?
Userthatdownvoted: Because of this, this and that.
My proposition is the following: Everytime you cast a downvote, a prompt of "Downvote Summary", similar to "Edit Summary", open up and you should write at least a character. No penalty should be issued to the person that would simply type "a", or something. But I think this encourages the aspect I told about: the downvote itself gives motivation, and the explanation says "motivation for THIS".
EDIT: Let me be more clear. Asaf showed me related questions, and on one of them, the following is the most upvoted answer:
No, I do not think the downvoter must leave a comment. Often I do, but to enforce it is a bad idea; it is already encouraged by the software for newer users.
An incomplete list why it is a bad idea:
-It is impossible to enforce that a meaningful comment is left, so it would just be some comment.
-There are legitimate reasons to want to downvote anonymously, for example, when the downvotee is know to be a difficult user one might not want to engage in a discussion with them.
-Multiple downvotes would lead to multiple comments. (...)
I aggree with the value of anonymity, and I aggree that a "spam" of comments is not helpful (and, of course, spammy). And I think it is good the fact that we can't impose a meaningful comment: a non-meaningful comment means that the downvote is non-meaningful. I was not clear in my suggestion:
Suppose that the "Downvote summary" opens up, and a "text block input" issues you to put some comment. You put anything, or a reasonable explanation. My idea is: all comments that are left stay in a "page", like the edit page, with some sort of "log", in which you could see all of them: and they would be displayed, anonymously, in order. The only downside of this idea I can see by myself is the fact that (maybe, I don't understand much about webhosting etc) this could create much more data to store, but I don't think this would be an issue.
Is this a bad idea? Why?
EDIT2: Scavenging a little, I found this answer that proposes something quite similar to what I proposed:
Which is highly upvoted, but I think that didn't get enough practical attention, I guess. What are your thoughts about this?