You mean exclusion of moderate to right leaning opinions right? That's my experience. If not (or even if so), I've seen lots of discussion on HN where everyone thinks their pet viewpoint is getting unduly picked on. That's probably as good a sign of neutrality as one can find (not that I think HN reader moderation is neutral)
Edit: maybe I'm misunderstanding the replies but my point is that many people seems to feel like the overall forum sentiment is against them. I'm not saying HN has a demonstrable left-wing bias. People seem to be trying to refute that for some reason