Early reddit treated the downvote the same way snapzu does. When the mobile site called them "agree/disagree", people were mad. " This isn't what the downvote is for!"

But gradually the admins were seen less and less (most admins I know of are gone) and they stopped propagating that idea. I think that what finally killed it is the large digg influx that weren't told about it.

To make it sure everyone is told about it, I think an explanation pop-up on your first downvote would do nicely.