Perhaps official downvote policies messaged to a user the first time they pass that would help too.

Anything with messages could be implemented by a bot account, right? That could be made without having to change the Less Wrong code itself.

Maybe we could send a message to users with guidelines on downvoting every time they downvote something? This would gently discourage heavy and/or poorly reasoned downvoting, likely without doing too much damage to the kind of downvoting we want. One issue with this is it would likely be very difficult or practically impossible for a bot account to know when someone downvotes something without changing the LW code. (Though it probably wouldn't require a very big change, and things could be limited to just the bot account(s).)