I also agree with #2, a single unpopular comment can ruin your karma, and seems to rapidly contribute to a rate limiter that impedes your ability to interact with the site. I tend to comment on spurts when I've got a bunch of free time. It's common to have a comment downvoted below 0 and end up with "you're doing that too fast" rate limiting. I'm not 100% that downvoted comments are the cause, but it seems likely.
I disagree with #3. The few mods HN has already work hard. I don't think we should give them more to read. I would rather remove downvotes, I don't see a point besides encouraging echo chamber mentality
And what do you do if most HN readers aren't like you?
But seriously, for number 3, what if the tallies for the reasons for downvotes are visible to the comment poster, as a form of feedback for that person?
2. Limited number of upvotes/downvotes, or downvotes substracting your own karma (perhaps not 1:1).
3. Mandatory justifications for downvotes (e.g. choose from dropdown menu), with this information publicly visible for each comment.