It would be good to make sure / very strongly encourage users to explain every downvote. The commenting can be brief but at least point towards the high level reason for downvoting.
I suspect there is no technical mechanism to do this in SE. But perhaps we can at least foster a culture that encourages this. Downvotes without explanation are pretty useless in my opinion.
How has this been handled in other SEs?
Edit: While there are questions, e.g. feature requests such as this one, where it is extremely clear what a downvote means, most technical questions won't fall into this category and an unexplained downvote then just becomes noise in my opinion. Seems to be a controversial and apparently well discussed SE topic though.