I like the system here where you cannot downvote until a certain aging process has occurred. Granted I’m not there yet, but not being able to downvote doesn’t impede the value I find on HN.
1. You can't downvote direct replies to your own comments
2. You can't see comment scores other than your own
3. Heavily down voted comments aren't hidden, just greyed out
4. New comments are initially sorted near the top of the thread, so they aren't immediately buried and have a chance to get voted on
5. Comments on older threads can't be downvoted
I wish you could also see the scores of direct replies to your comments, so you know if the replies are popular and merit being addressed.
As it stands now, if you see 2+ replies, and they aren't greyed out or responded to, you have no idea if they're heavily upvoted, or just got ignored.