The standards for what's germane to Hacker News have gotten looser. TSA is only the most recent example. What's especially toxic about this fact is that you don't notice it until it gets really bad. That's because most of these stories have nerd-structured narratives, involving tradeoffs and logic and subtext and affordances for contrarianism, which bait commenters. Having participated in a TSA discussion (for instance), you become socially committed to the idea that they're relevant to Hacker News.
Hacker News has become much more self-referential. All due respect to 'lionhearted and 'DanielBMarkham and 'jacquesm, but there have been many stories voted to the top of the site on content that wouldn't stand had they been written by an "outsider". There's a clear name-recognition bias. That's not the author's fault (it's their blog, they should write what they want), but it does make the site feel insular.
I'll go out on a limb though and assert that insularity is something 'pg cultivates. My most recent cue on that was his encouraging response to "Offer HN".
Like it did for Matt, Hacker News has killed any desire I have to write standalone content. I haven't blogged in over a year. A book idea I was tossing around has been dead for longer. Hacker News fills the same psychological place for me that Usenet did in the 1990s, when I also didn't write a lot of standalone content. Now, for me, this is actually a good thing; I dove into HN while fleeing the "blogosphere". But I can see it being a problem for someone else.
Having said all that: I get tremendous value out of HN. I've met tons of people running startups, I've done business with some of them, I get to carry on long-running conversations with people like Patrick McKenzie and Colin Percival, I've hired several awesome people off the site, and I'm still impressed by the newcomers (for instance, go read 'carbocation's backlog of comments on biology and medicine).
Perhaps I'd like to see people a little quicker with the "flag" button; perhaps I'd like to see the site tuned so that flaggers can more easily win the race against thoughtless up-voters. And it might be nice if we could take a break from blog posts by long-time contributors; maybe we can switch to a "best-of" 'lionhearted mentality, instead of a "today's" 'lionhearted mentality.
But, while it sucks to lose Matt (he seemed like one of the more no-bullshit members of the site), I'm not as alarmed as he seems to be about the decline of HN.