According to privilege page for down-voting:
Downvoting should be reserved for extreme cases. It's not meant as a substitute for communication and editing.
I happen to take this seriously, and always make sure to offer an explanation with every downvote I dish out. By the same token it bothers me quite a bit when I get an anonymous, no-explanation downvote on a question I've carefully crafted. While I think it's more than fair to ask for an explanation, I usually get heat for it- the prevailing attitude seems to be that questioning a downvote is "noise", "pointless", and generally bad form.
I would prefer the system required a comment on every downvote- it would be more constructive than the current "you know what you did" approach. But seeing as how we're relying on ettiquete, is asking for an explanation off limits?
P.S. I'm ready for the rain of ironic downvotes- I'm taking this for the team...