What about a little non-binding banner that pops up when someone down-votes saying, "Thanks for the feedback - please take a moment to explain your reasons for down-voting in the comments (optional)." or some such...
Unexplained down-votes leave me with such a hollow feeling.
(Likely this should be migrated to meta.SO - maybe its been asked there - I don't know I'm on a phone connection and can't check.)