You're right this is a cycle I've seen time and again online. I've been using the Internet since before the web, and seen this cycle again and again... newsgroups becoming popular, the negative argumentative types moving in, folks who weren't that way getting dragged into the negativity (I know I've been dragged there myself), then they just become festering shells of what they once were as the nice people move on and just the negativity remains to harshly parody the original community...
The same has happened with web community after web community I've enjoyed only to see them get popular... slashdot was good once. Heck, there was even a time when you could get a civilized conversation in TechCrunch comment threads.
Slashdot's current state really breaks my heart. Even with the occasional drive-by troll or the "apk" lunatic (what's his deal anyway?) things were tolerable, but lately it seems some of the older posters have lost interest and moved on (maybe to HN? Ha!) or have reverted to petty squabbling and deliberate obtuseness. I mostly go there to read comments from the veterans who've stuck around over the years and only recently bothered with creating an account. Thankfully there are still nuggets of gold among the threads.
There's a tricky Goldilocks zone when it comes to any forum and the biggest challenge is not treating it as a personal blog. You share ideas, yes, but the very platform demands you give up some control. Of how your ideas are perceived, what feedback you get and even weather those ideas are welcome in the first place. And all communities change... away from a Beowulf cluster of X and into the cloud, I suppose.
Trying too hard to hold new members to the light of older burning candles will cause them to flicker out or if you just let them burn willy-nilly, you risk starting a fire.
I'm hopeful we're not all doomed to walled gardens the likes of Twitter and Facebook just to avoid negativity raining on our parade (of course even they're not devoid of doom and gloom either, but "blocks" do give you an umbrella of sorts).
Slashdot used to be full of field experts discussing ideas and tech, you could see discusions on there from genuine industry and academic luminaries. Most of them have long since left as it devolved into negativity, trolling and fanboyism.
I stopped going there when it became obvious the the first posts on many topics were no longer genuine. Not only were the subject-matter experts gone, but the 'discussion' was being led by paid reputation managers who had prepared statements on various things.
The same has happened with web community after web community I've enjoyed only to see them get popular... slashdot was good once. Heck, there was even a time when you could get a civilized conversation in TechCrunch comment threads.
So, with the obvious caveat of http://www.despair.com/dysfunction.html ...I sincerely hope that HN doesn't end up going down that road too.
Long winded way of saying I like your comment and I'm gonna try to take the advice.