As there are hundreds of comments on the issue of users down voting a question without leaving a comment - I believe it must be a serious issue to many users.
Since some users don't feel it is important to explain why they are down voting a question, the OPs are unable to learn anything from the down vote... stifling the growth of useful questions on StackExchange.
I realize that many down voters are afraid of retaliation down voting - (and many trolls should be)... but I believe there is a viable solution that would solve the problem without opening up anonymous down voters to such retaliation.
Why don't we?...
- Have the system automatically add a comment for every anonymous down vote that simply states, "down vote with no explanation - @anonymous"
- Have the system internally track the anonymous commenter's userid
- Allow other users to down vote such an anonymous down vote comment and have the system apply the down vote penalty to the anonymous user's account
- Not allow the question's OP to down vote such a comment
Question down voters can still remain anonymous and safe from retaliation voting if they want to... but their reputation can now be made to suffer, just as if they actually did comment about their down vote, if other users disagree with them.
This would also overcome the issues concerning an unenforceable "requirement" that a down voting user add a comment that explains their reasoning... without requiring moderator monitoring and intervention to make such a solution viable.
I believe this solution would stop a lot of trolls and lazy commenters - and really aid in improving the quality of questions by getting users to provide valid feedback, thereby educating the OPs in how to create better questions. All while getting StackExchange back to a much more user-friendly environment.