Yes. A large part of it is size. It's hard to discuss this without sounding like an elitist or a stuck up braggart, but that is not my intent. I'm also going to use Reddit for comparison, and it might sound like I'm slamming Reddit, but I am not. I've been there for over four years, and spend a lot of time there (way too much time). If it helps, while I am typing this I'm kind of watching and listening to "Finding Bigfoot" on Animal Planet. I think admitting that should prove I have no illusion that I'm superior...

I'm a reasonably bright person. On objective tests that are usually considered reasonably good indications of intelligence, I do reasonably well (99.6 percentile on the LSAT for instance). I managed to get a bachelor's degree in mathematics from a tough school (Caltech).

I find I prefer spending time, in real life or in virtual life, in the company of other bright people. In fact, I'm happiest when I'm generally outclassed by the people around me, but they are not so far ahead of me that I cannot understand what they are saying and make valuable contributions of my own. I do not like to be one of the smartest people in the room (real or virtual).

Reddit, at least the technical groups I had frequented when I signed up there over four years ago, was like that. But now those groups have 500k to 1 million or more readers. Just doing the math, you can't put together a group on an open forum like Reddit that is that big and still has enough smart people for me to be comfortably away from the top.

Such large groups tend to have many people who can't distinguish on their own between someone advocating a repellant position, and someone disagreeing with a flawed argument against a repellant position. It gets frustrating when someone says something, especially when they are advocating a position you actually agree with, that turns out to have some factual flaw, and then you spend 20 or 30 minutes working on a reply, and then you are massively down voted. On a lot of subreddits now I have stopped researching my comments--I just go from memory. If someone wants more information, or citations, they can use my comment for ideas to start their Google search.

HN when I signed up, a little under two years ago, was ideal. Nearly everyone seemed at least as smart as me, and I started recognizing a lot of people clearly my superior. It was like being back at Caltech. A down vote on HN actually meant something--if a comment of mine got a down vote it invariably meant I had done something really dumb and I'd learn from it.

And on HN you could post a devil's advocate argument without having to first explain that you are playing devil's advocate. You could take a contrary position just to get a good argument, and people would welcome it as long as your point was well written and logical.

As HN grows, it invariable is going to become more Reddit-like, but that behavior is more damaging here. Reddit has subreddits. You can skip the cesspools of stupidity like /r/politics and /r/atheism (my vote for the most disappointing subreddit) and at least get some good stuff in /r/programming (one of the few reddits to stay mostly reasonable despite growth) or /r/physics or /r/math.

HN doesn't separate things.

I'm using an add-on to inject custom CSS on the site, and I use that to highlight down voted comments in bright red so they stand out. I up vote any such comment if I can't see a good reason for the down vote. I've been finding lately that I have been up voting 90% of these comments. I up vote these even if I think the opinion in the comment is completely wrong, as long as it is expressed well and respectfully.

I don't know if this decline can be stopped, or if it is inevitable fate of every public discussion forum. One thing we can do is back off on things where they are adequately covered on Reddit AND the Reddit discussion is as good as the HN discussion. I've taken to flagging most of the political submissions unless something stands out to distinguish them. I've avoided this for a while, but in almost all of the discussion I've not seen any sign that the discussion here is better than that on Reddit.

Recall the advice from the HN guidelines:

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On-Topic: Anything that good hackers would find interesting. That includes more than hacking and startups. If you had to reduce it to a sentence, the answer might be: anything that gratifies one's intellectual curiosity.

Off-Topic: Most stories about politics, or crime, or sports, unless they're evidence of some interesting new phenomenon. Videos of pratfalls or disasters, or cute animal pictures. If they'd cover it on TV news, it's probably off-topic.

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Perhaps these could be tweaked a little. For the "Off-Topic", maybe "on TV news" could be expanded to "on TV news or on a large subreddit".

Again, sorry if any of this sounds elitist.

PS: they did not find Bigfoot.