I generally downvote nodes for these reasons:
- Excessive rudeness
- Answer that isn't at all relevant
- Bad advice
- Unreadable one-liner given to obvious novice, without accompanying documentation and warnings
In each of those cases having one comment on the node that points out it's flaws is sufficient. We don't need 20 people saying "That was rude", or suitable responses to the above mentioned list.
That said, it would be nice to know why people vote a node down. In most cases I can see the problem but occassionally a node has downvotes for no reason that is obvious to me.
One way to address that issue would be to provide an optional one line text input on the voting line where you can record a concise comment. This line would not be visible to anyone but the node's author, and would be anonymous. This way constructive feedback could be given. It could also be treated as an anonymous message for the author, that way they wouldn't have to check each node to find out what the issue was.
On a somewhat related note I would love to see the vote distribution on my personal node list, so I can see at a glance how many down/up votes each of my nodes had. When I get an XP loss message it makes me curious which node got downvoted, so I review the most recent 10 or so.