As it seems to me, some topics are automatically down ranked by the system or by moderators.
Today some very important person won a rather important prize (I don't tell the names to prevent automatic down ranking of the system). I and (as I saw later) at least one other person posted the news that was in the press.
But just one or two minutes later, both posts where vanished. I also looked 100 or so posts deeper, gone!
I also posted a similar message again, since I thought that it was a system error. The same effect -- after less than one minute, the post vanished from the list. It is very unlikely, that the post aged so fast by the normal ranking mechanism. The other post also got one up vote from me, so it is even more unlikely that it vanished from aging.
I don't know, why such posts are suppressed and what is going on. It is at the border of censorship in my opinion. Of course it could be some mistake, because similar news where around for some time ago -- but this news, I am talking about was definitively new and relevant!
It is true that some topics are penalized by the system, and some are automatically killed. Browsing with "showdead: yes" will give you a feel for which these are. Most of them are straight up spam, although some are good posts from 'hellbanned' users. The moderators kill or penalize things as well, but more commonly it's flagging by regular users that pushes hot topics off the front page. Yes, this can be viewed as a form of censorship, although "active moderation" might be more exact.
I'm less certain about the silent hellbanning, but I think overall the degree of moderation is a positive. While there are costs, it's done an admirable job keeping HN usable and readable as it's grown. Not all news needs to be on the same site. If a post is divisive and unlikely to create useful discussion, perhaps it's better to keep the flames down by keeping it off the front page. While I'm sure there is some moderator prejudice toward specific topics (both conscious and unconscious), I think this standard gets applied fairly evenhandedly.
Email sent to "hn@ycombinator.com" will usually get a quick response to questions about specific situations.