Sometimes there are nodes that are voted down for reasons that are obvious to the voter, but a mystery to the node's author.
When reading Put your mouth where your money is? it occurred to me that one way to clarify the rationale behind the downvote would be to have a one line text input on the same row as the vote buttons. The text input here would be sent as a message to the node's author, and not be visible to other users. You could also send positive messages of course. It might be nice to make this message anonymous to keep the spirit of voting intact.
Due to what could be a large number of messages being sent it might be good to categorize the messages generated and provide an option to ignore them.