PG already commented somewhere that he's experimenting with a number of solutions to this problem, and will hopefully come up with something as quick and convenient as the downvote but with the depth of requiring an added comment.
Uh, yes, that's precisely the opinion my whole comment was objecting to. How can that not have been obvious? Did I edit the baby out with the bathwater? Let me try again:
There are many occasions when a silent vote is the most appropriate feedback, for two reasons: (1) one doesn't always have time to type out a comment; (2) one's comment, if typed out, might not add value. There is a psychological threshold above which an explicit comment makes sense and below which it does not make sense, and trying to mandate otherwise will not work: it will only result in a lot more tedious comments. Indeed, many suggestions people are making strike me as rather schoolmarmish. There are few better ways to suck the life out of things than to make them mandatory.
I would absolutely agree with this statement.