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.…
This isn't an HN problem, it's an Internet problem. pg could probably do some things to ward off dilution, but it won't last forever. Interestingly enough, Something Awful seemed to be pretty damned successful in this regard by doing the unthinkable and charging people to post there. I'd hate to suggest it as a solution, but it has been effective in the past.