I think it would be useful and clarifying; and may it be anonymous.
Down votes already contain stock explanations:
For a question: "This question does not show any research effort; it is unclear or not useful."
For an answer: "This answer is not useful."
I think these are pretty clear. Simply hover your cursor over the down vote arrow to see them.
I don't feel down votes are "superfluous" in a community driven site. Flagging for moderators should only be used if something violates the TOS or is inappropriate. And while meta can be used to discuss issues with a question, it shouldn't be littered with commentary on every question or answer a user feels is "not useful".
The down vote allows a user to show their opinion without any great impact. I mean it is only a -2 to rep. For someone simply browsing the site, seeing some questions/answers with lower votes clearly indicates that other users don't feel the question/answer useful and they may wish to pay more attention to those answers/questions with higher scores.
You can't force people to comment when down voting. There's already the pop up urging users to comment, but if they choose to ignore it, they have that option.
The down-vote functionality is the result of over engineering, in light of the other tools available it's superfluous .
It's enough for a post to have a score of zero if it is not useful. If the post is bad or inappropriate users can flag it and moderators can remove it. If a user finds that they have a problem with a post they can post a comment against the post or raise their concerns on the meta site.
The down-vote by virtue of it's very nature can only have a negative impact.