I think the difference between reddiquette and the rules here are that here it is actually explicitly spelled out that down votes are not for things you disagree with where are on reddit and this is what always annoyed me is people saw reddiquette as a suggestion and would always argue it as such when ever this issue came up.
I think the solution not to give more transparency to the community as a whole or to put some cost on downvoting but to give more information and transparency to the mods and other community leaders. That way the people who made the rules for each tribe and who have the clearest understanding of how they should be enforced can look at what is occurring in there tribe and do something about it if necessary.
Edit: As an addendum to this if my above suggestion was implemented an easy for mods to enforce it would be to add to the rules of their tribe that all downvotes must accompanied by an explanation that way when mods are needing to deal with a complaint about down votes they can easily see if someone has downvoted and not left a reason and they can deal with it as they see fit. This is in reply to the suggestion to force people to give an explanation when downvoting.