I have been around HN for about 4 years now (I lurked before I created my account) -- there used to be all sorts of interesting articles on the front page (not just about programming, but genuinely interesting things like music, biology, psychology etc). Now, every third story is about NSA/Snowden, Openness, or someone's blog about how XYZ is bad and we should all do ABC instead. The front page typically has maybe one post that I care about these days.
Comments similarly started lacking in quality, or maybe the really vocal minority are just louder than everyone else. You're right that slashdot comments are way worse (jokes aren't frowned-upon as they on HN, so a lot of discussions degrade to cheap jokes), but the articles are moderated by a handful of people and are typically more interesting.
It's simply a function of a user-run moderation system that reaches a critical point, after which is it logistically impossible for the "good few" to moderate out all the bad ones because their votes count the same. We saw it in Slashdot comments, Digg and Reddit, and now we are seeing it here.