Should we require users to provide feedback on the -1s?
Deeeeefinitely not. Feedback is great, but not everyone is great at phrasing why they've voted (politely), or others may have already pointed out the problems (so more comments is just more pile on). I'll just quote this earlier answer of mine:
You're not required to leave a comment, nor will you ever be. On Meta Stack Overflow (the Meta for the network, basically) this issue has been retreaded time and time again. You're encouraged to explain what's wrong with a post, but not required, and all efforts to require comments on downvotes have been struck down.
If you're going to comment, and I generally suggest you do, focus on what's wrong with the post, not the fact you downvoted*. This focuses the author on what's wrong, not "hey some jerk downvoted me WTF". Generally I try and leave off the "-1" or "I downvoted this because..." when leaving a comment. Just explain what's wrong. This has the extra benefit of reducing the likelyhood that you'll get someone angry enough to revenge downvote (though some automated scripts are in place to limit the potential damage of revenge downvoting).
Whether you downvote or not, if you see something wrong, explain what it is. But what's more important than voting is the content, so think content first. At the same time, don't let that stop you from downvoting bad comment if you don't have time to explain it in a comment.