To me, the decline is an obvious Eternal September-like effect. The more popular it became, the more it attracted trolls, people with political agendas and other destructive forces. I think that sort of thing is as inevitable as programming languages ending in feature-bloat. I expect the same will happen to HN, even though I think the mods have done an incredible job so far and even though the atmosphere has shifted a bit I still enjoy the discussions here.