I'd like to see anonymous upvotes, but when you downvote you have to leave a comment or it says 'downvoted by UserName' (maybe only for negative-karma comments).

It turns a downvote from 'I disagree' to 'I disagree and everybody else you should too'. You have to be willing to risk your own karma to take away somebody else's.

For instance the way HN works now afaik, somebody could post 'me too' comments to get enough karma to downvote, then turn around, downvote all posts supporting some idea they dislike, and there would be no way for other posters to correct this or know it was happening.