An interesting aspect of HN, I think slightly different from some other communities I've been in, is how this sense of decline has been there pretty much since the beginning. Here's a discussion thread from HN's first year worrying about the decline: http://news.ycombinator.com/item?id=60767

I suspect part of it is that it's been a very meta/self-aware experiment from the start, whereas many communities sprout up accidentally for various reasons, and only later get more self-conscious about it.