One can always claim this about something that hasn't happened yet. But it's already been two years and 15x growth and the sky has not fallen. So you could just as easily argue that what has saved us so far (killing fluff stories, insisting on civility in comment threads) will continue to work.
Philosophically HN is different than reddit and Digg in that the person who runs the site is active in shaping the ongoing conversation. Reddit and Digg ultimately became for-profit companies and in doing so, embraced the democratic method of policing their sites.
HN on the other hand is somewhere between an oligarchy and meritocracy; and can only work that way because pg doesn't appear to want to monetize it.
My point is that reddit and Digg are far more similar than reddit and HN or Digg and HN.
The level of conversation on Reddit didn't decline because the founders were apathetic. They still care a lot about the site. But they are committed not to censoring anything, either comments or stories, and that tends to make the site something of a free-for-all. A massively successful free-for-all, incidentally:
For most users the change in the character of Reddit couldn't accurately be called "decline," or their traffic wouldn't still be growing so fast. It just seems like a decline to the small minority that includes us.
I'm not suggesting that the "sky will fall", I'm suggesting that it is inevitable that this site will change if it continues to grow, user count wise. To suggest it hasn't evolved and changed from it's original state is disingenuous really. (Not that you are doing that, specifically)
Digg and reddit have turned into a glorified dorm room because that's what their average user is, a male college kid. Clearly, many people do feel the user base of this site changing - we wouldn't be commenting on this thread were this not the case.
It's not a good/bad thing, as it means you've chosen to broaden your base as opposed to remain niche. There's nothing wrong with that. But at some point, you hit the magical land where the average user demographic has changed so much that the tipping point is passed.
Digg and Reddit also make no attempt to welcome new users and to bring them up to the standards of the site.
The people who've been here for longer than a few months all try to get new users up-to-speed. As a result, HN's wavered once or twice after really big traffic hits, but I haven't seen noticeable lowering of quality yet - that may be a sign that things will still stand.
(I will note, however, that a lot more disagreeable trends have shown up over time: OS-bashing, obsession over one or two services in a given field. It's not awful, and it can always be avoided, but it's present now.)
It may seem casual, but remember that this is a benevolent dictatorship: nobody has any illusions about this being anything other than pg's project, and that's a really good thing for the future. It means that there's one unifying vision for this site, and it's one that's powerful enough to keep people here, and it's a good enough one to make it worth being in. If something goes bad, he has control over fixing it.
When he holds the keys to the user accounts and the stories, he can react pretty quickly. He's shut off accounts temporarily (once to avoid an edit war over a title), and during the Obama election, a lot of stories ended up dead very quickly. That's what kept the site contained.
Not for the same reasons - Digg does it for people gaming the system. Here, that kind of thing is exceedingly rare, and HN doesn't ban most of the accounts, it just auto-kills their submissions. HN takes a unique approach to why accounts are banned. If you troll or get into flamefests, pg very often takes time to tell you you're doing poorly (that happened to me last summer). If you keep doing after that, then you get disabled - I think.
It's a system that's very hands-on and it's kept this place scaling really nicely.
When I took freshman philosophy, our professor told us about induction ("it's always been this way, so it will continue to") and anti-induction ("it hasn't been this way yet, so it probaby will soon"). He then explained that the argument for induction was that it's always worked before, so it probably will in the future, and the argument for anti-induction was that since it's never worked in the past, it's bound to this time. :)
news.ycombinator is successful because it's first the web forum for YCombinator program participants. "Trimming" works (civility and killing fluff stories) because what remains is a community of vetted YCombinator members and their friends.
One can always claim this about something that hasn't happened yet. But it's already been two years and 15x growth and the sky has not fallen. So you could just as easily argue that what has saved us so far (killing fluff stories, insisting on civility in comment threads) will continue to work.