It's very demotivating if your correct answer is downvoted by some lazy unconcerned passer-by, especially where you have spent about one hour with creating the working solution and without any reasoning!
Imo downvoter should be obliged to anonymously provide exact reason for his downvote, and so that such an explanation will be good feedback or good hint for the answerer/asker.
When the downvoter clicks on the
downvote arrow, then a popup could raise, with a few selectable predefined downvote reasons, just like it's required when reviewing suggested edits, e.g.
Reason for the Downvote:
o "this answer is shoddy or useless (default)"
o "this answer is out of topic (unrelated with asked question)"
o "this answer is obscure"
o "this answer is incomplete (not works in all situations)"
o "link only answers are not good answers"
o "the explanation in this answer is wrong"
o add your own anonymous coment: 
[Confirm the downvote]
Currently the popup message
"Please consider adding a comment if you think this post can be improved" is shown after downvoting, but it's naturally ignored by majority of downvoters, because it's not anonymous.
:: UPDATE ::
Or simpler alternative could be this: after downvoting there can be shown a simple popup:
"Please consider adding your commentary explaining the downvote:
[ ... ]
<ADD ANONYMOUS COMMENT> <LEAVE IT UNCOMMENTED>