Excellent question, I too think that the comment and perhaps post quality has somewhat deteriorared. To me it's the inane comments that don't add anything which bother me the most. And I don't want to start downvoting everyone for doing that but still, I'd much rather read some deep and thought-provoking observations than remarks such as "that's interesting" or even worse, puns.
But I guess this is what happens with all services when they start to get "main-stream" and start to become diluted by the masses. Without sounding too elitist, it's not all too terrible but you can see the difference between older posts comments and new.
What concerns me the most is a wave of people who don't like learning. It's the best part of our jobs! People are complaining about new languages, frameworks, platforms, features here more and more.
There's always at least 1 post hanging in a top that is about "You don't really need X", which really says "If you're going to keep doing next 5 years what you've been doing last 5 years, don't bother", which as I suspect some read as "Relax, you don't need to bother about this tech, just memorise a few sentences to sound smart if asked about".
It's HN. There's something new? Cool! You managed to serve 1000s users per second from linode $20 node? Awesome! How? What tradeoffs you had to do and where? You have a good idea on how to better make choices about your tech and created some nice decision tree chart? Thank you. Hope it's on github, we'd like to collaborate.