Downvoting on questions and answers are an important part of the StackExchange network. It's there to show the communitys total voting. But to be able to ask better questions, and provide better answers, I would like to suggest a comment to be added to the downvote.
The down vote is someone who disagree with the post, and it would greatly help if a downvote was accompanied by a -1 comment. This makes it easier for the poster to know which part of the question/answer the downvoter disagrees on. The poster may then edit the question/answer to make it more in line and maybe improve the post.
An upvote is different, since the voter agrees on the post, and no comment is therefore needed.
Not forcing - Encouraging
Thanks to Daniel's answer, I need to clarify myself. the intention is not to force users to post a comment, just to encourage them to add a comment to give the poster feedback of what was downvoted on.
This has also been discussed on StackOverflow Meta in the post Encouraging people to explain downvotes.