I confess. I have a quick trigger finger. On more than one occasion, somebody on HN has said something that caused me to vote them down, sometimes, even before I finished reading their comment. On other occasions, I've read the comment, but hadn't parsed it properly. OK, and a few times, too much coffee has made me actually miss the up arrow, and hit the down arrow instead.
On all these occasions, I have felt a little guilty. Not enough to lose sleep, but still, if I had the ability to go back in time those two seconds and undo the vote, I would do so.
My thought is that, if I down vote somebody, I should lose my down arrow, but my up-arrow should remain, in case I have a change of heart. Similarly, I suppose, I may change my mind about an up-vote, so the reverse logic would also apply.
This would save me the trouble of actually carefully digesting what my fellow HN readers have written before voting, while still preserving my guilt free state of mind.
Master the Zen of HN
Vote up posts or comments for the good of HN rather than as a reflex action to something you have just read.
In my opinion HN would be a better place without a down vote on comments - the simple choice between an up-vote and no-vote is all that is required - a moments contemplation - a choice made - then move on.