Here is an idea: Instead of deleting "down vote", instead, add "yay!" and "boo!" links.

A user can see the "yay!" and "boo!" scores of any recent post or comment.

"Yay!" and "boo!" scores do not effect karma or otherwise effect HN privileges.

(The idea is to allow the exchange of "push a button" feedback that, in some sense, doesn't count. Up/down votes can still work as always.)