I came across this question recently and asked the OP to explain some notation (see my comment below the question). As the OP was addressing my concerns, there was suddenly a down-vote (along with the resulting $-1$). Now, this is awful for at least two reasons:
there is absolutely no feedback for the OP as to why a fellow MO-user had a negative perception of his question, and
it now looks like I got annoyed by unexplained terms and down-voted even as this poor guy was addressing my concerns and defining his terms.
So, here is my question:
How can we encourage people who down-vote questions -- even bad, unsuitable questions -- to either leave a comment explaining themselves or to up-vote an explanatory comment that is already present?
This type of hit-and-run is really bad for our site and the way it is perceived by those not already using it regularly. When there are down-votes -- but not votes to close -- it is likely that low-reputation users are responsible. There is at least some accountability for voting to close (since your name is revealed upon closure), but almost no accountability for a random down-vote. We really should work on creating a better deterrent to this type of behavior.
There are several significant problems with enforcing a comment for every downvote:
There is no good universal way to determine if a user already left a sufficient explanation, so a bad post will accumulate a lot of nearly identical comments that explain why it is bad. This creates a lot of unnecessary noise and also might look even more hostile to new users than unexplained downvotes.
There is a significant number of people that don't take criticism well, even if it is constructive. And if forced to leave a comment for every single downvote, quite some users are likely to resort to more direct and blunt comments. This would very likely increase the cases of revenge downvoting and conflict in comments.
It would almost certainly discourage downvoting and decrease the total number of downvotes. Downvotes help to remove bad questions from the frontpage, and to properly sort the answers and put the bad answers greyed-out at the very bottom. Discouraging downvotes might have a bad effect on the site due to that.
That said, commenting why a post is problematic is a good idea in general. I would personally avoid including that you downvoted, but explaining a user how they can fix their posts can be very helpful and should be encouraged. Explaining downvotes should be encouraged, but not enforced by the software.