My personal problem with HN, is that every time I dare to write a comment that is not positive about something, I get downvoted and in most cases the downvoter doesn't care much about explaining it, even when my comment spawned an interesting discussion.
It's very annoying that 100+ karma users could act as really good trolls, they could just go around and downvote everybody without ever loosing karma, but costing the commenter karma.
Don't get me wrong I really enjoy the HN community, there are a lot of bright people here and very good content and discussions, it's just that the system discourages you to have your own opinion on anything, because if you write something that a 100+ user doesn't like he can simply downvote you irresponsibly.
TL, DR: I don't know maybe it's me, maybe I have too radical opinions on stuff. But overall I feel that the downvoters should loose some karma too, and maybe force the downvoter to write a reason for it, to make sure that they downvote responsibly.
PS: I am not saying neither that I'm always right nor that I never deserved the downvote.
It seems to me that HN is relatively neutral in terms of your opinion; People here seem to avoid the vice of downvoting based simply on a difference of opinion, at least in my experience. The community generally seems to award karma based on how thoughtful and carefully-constructed your comments are. I have one one occasion been downvoted into oblivion for politely and carefully expressing an opinion that is generally morally unpalatable, but someone came to my defence and I eventually ended up with a small amount of positive karma for the comment.
I think the easiest rule of thumb is to try and be the opposite of cable news. The calmer and more dispassionate your tone, the more detailed and precise you are in your reasoning, the more carefully you reference reliable sources, the better your chances of being strongly upvoted.