Ah yes, the cycle of website life. * Hot new community forms at Site X. * Site X residents refer to themselves as the New Wave of whatever. Much better than older Site W because of features/members/dynamic/demographics 1, 2 and 3! * Site X's reputation spreads to former hot new sites T, U, V and W. Site X begins to attract more and more new users. * Site X denizens begin linking articles at T, U, V, W and vice versa.…
Interestingly, this isn't a problem related to any particular category of site. Hell, look at 4chan: bitching about newfags and how /b/ isn't good anymore, how /b/ was never good, different factions with different ideas on how to fix the problem (or what the problem is, or if there even is one), spinoff sites where the early adopters migrate to... This isn't an HN problem, it's an Internet problem. pg could probably…
Sometimes they move offsite instead, usually because the site admins didn't actually like the topic enough to make more forums for them, or because the admins banned all the users because they had formed secret IRC cabals conspiring against them.
The last stage is that the admins change the forum CSS so it's unreadable. I haven't figured out that part yet.