I've seen more than once some downvotes of (easy) questions with no comment. I think it would be fair to force a comment to be filled out for someone who downvotes. That comment should consist of at least four words.
This has been discussed endlessly on the main meta:
- Proposal: Require anonymous comment with downvotes
- Down-voting on questions - Require a comment below a certain rep?
- Why not allow downvote only with at least an anonymous comment?
- Different implementation of downvoting: require comments
- Penalise Downvotes without Comment / Reward Downvotes with Comments
- Change Down Voting to Require Comment for Why & Change Flag to Report
- Is there a way to require an explanation for a downvote?
- Proposal to solve the Uncommented Downvote problem
You can read those for information on why this has been declined in the past, but it comes down to the fact that downvotes are supposed to be anonymous; the proposal for anonymous comments went poorly because if you want to leave a comment you should just do it, and on the rare case that somebody goes on a downvoting spree of their own the system will catch it
I disagree. A downvote on a question has a built-in reason: "This question does not show any research effort; it is unclear or not useful."
If people want to provide greater reasoning and assistance to the questioner, great. But a downvote alone is fine. Voting is important to the functioning of the site, and encouraging that is more important than making hoops to ensure that each vote is justified or explained.