HackerNews has one of the best downvoting rules I've ever seen - you can't downvote someone replying to you. I think that simple change massively changes the way karma works.
They also arbitrarily don't allow you to reply to lots which is annoying. I often have follow-up questions (legit ones, not comebacks or other crap) that I can't do anything about :(
But I agree, its generally terrible etiquette to downvote something someone has contributed to you if its goodfaith and also, assuming your thing is visible people are gonna see it and your interests are linked so its just silly, bottom-line
From what I can tell, all the karma thresholds are dynamic and probably only knowable by admins. If nearly 1000 isn't enough to avoid rate limiting then they sound pretty aggressive.
From my perspective HN's approach seems to do pretty well at mitigating bad behavior, but might be a little too hard on newcomers and casual users.