I never see anyone complaining when they receive an up-vote without an explicit explanation...

As Iain is fond of saying: *"No one who needs to, reads anything least of all documentation."*   
Many first time posters skip actually reading the the [site introduction](http://serverfault.com/tour) and the [how to ask](http://serverfault.com/questions/ask/advice) pages and that is unlikely to change. In that light a mandatory/automatic explanation for a down vote will be mostly useless. 

At best we remediate that for users who care, which is why I make use of the StackApps [AutoReviewComments](http://stackapps.com/questions/2116/autoreviewcomments-pro-forma-comments-for-se) extension and insert the following comment whenever I see an user actively questioning the reason for a down vote:

> By philosophy and design votes are anonymous and **neither voting [up](http://serverfault.com/help/privileges/vote-up) nor voting [down](http://serverfault.com/help/privileges/vote-down) requires any mandatory explanation**. The tooltip that appears when your mouse pointer hoovers over the down button states: *"this question does not show any research effort; it is unclear or not useful"*. Also questions can attract a down vote when not [well written](https://meta.serverfault.com/a/3609/37681), not quite [on-topic](http://serverfault.com/help/on-topic) or missing details.


 That minority might improve their current question or will ask better questions in the future.