The quality has not noticeably gone down. There are a lot less technical articles now than I feel like there used to be, but that might very well just be because I've gotten more technical, and now everything seems like an opinion.
I think it's because HN is actually willing to defend itself. We used to have days where everybody would ONLY submit articles about some obscure programming language (although I have forgotten which one it was...not arc, I don't think...scala maybe? I think it started with an S).
We don't seem to do that anymore, but I think the culture is still there. People are willing to tell you to leave if you're not a good member. That's really important.