I never left slashdot. I just stopped going there. HN was a big part of that. I wanted my tech news to be thought provoking, funny and innocent. I had plenty of sources for "real world" news and I wanted tech to be an island away from that.
Slashdot became more about the legal issues surrounding technology than about technology. It had a militant, fanatical vibe that soured the taste of its brilliant gems.
HN is starting to feel like a place where activists hang out. The topics are certainly important and worth discussing - but the tone takes away from the lightness and fun of technology. It's like eating cheese and drinking orange juice at the same time. The two are good on their own, but they don't go well together.
I think this reflects a real-world trend in what's relevant to "hackers" right now. The financial aspect of the whole technology industry really seemed to take off after the Wall Street meltdown, after other financial avenues darkened (remember all those articles a couple of years ago about "why we're in a bubble/are we in a bubble?"). Right now, a number of legal issues are impacting technology (software patents, NSA spying, etc) and hackers are unsurprisingly interested in discussing them.
I don't think these are necessarily bad trends. I think you're seeing a bit of the maturing of tech industry and you're seeing that reflected in the discussion. But there is still a lot of great technical discussion on the site (the front page right now has a great story on a scanner bug, a compilers blog post, a theorem-prover as programming language article, etc).
And at the end, what happened to Slashdot is that reddit happened and all the smart people left, and what happened to reddit is that Hacker News happened and all the smart people left. Until there is a credible alternative to HN, I think you'll still see a lot of signal, even if there is more noise than there used to be.