I very rarely downvote.
I think the downvote option is probably needed but it can be problematic - it carries different meanings with subtle differences:
- A way to signal disagreement with a comment.
- A means to flag a comment as clearly misleading or incorrect.
- A way of signalling a 'low value' comment (obviously subjective).
- A way for the downvoter to simply express his or her displeasure at what has been posted regardless how reasonable the comment is. (This probably happens more often than people care to admit it. It's subtly different than just disagreement.)
One might argue all the above examples are types of 'disagreement'.
An idea:
Clicking downvote adds the following line to the end of a comment: 3 readers disagree or 3 readers downvoted. Would this make readers pause before they hit the downvote button? (And is that a good or bad thing?)