My experience is that HN's Overton window is probably on average 15-20% larger than most forums. That's not uniform across all topics though. So if you skew toward a particular set of topics it may feel like a typical forum, or even in some ways more constrained.
My issue is it seems like something has to only be a bit controversial to be completely hidden from everyone. There was the recent DF article about how Gruber thinks his articles are being artificially shitlisted and I can't help but agree? I don't necessarily think the mods have their fingers on the scale, but I wouldn't be surprised if the algorithm works in a way where if enough people flag something it gets automatically hidden, and there's enough people who see DF and automatically flag it that those blog posts get hidden every time.
My feeling is it also depends on weekday vs. weekend, and on the time of day (or night).