There are many great technical boards indeed, but if you know of one that's as eclectic as HN I'm all ears. It's the only forum I've found where I've had interesting discussions with intelligent(often more so than me, which I crave) peers on pretty much every major or minor interest I have. And the "forum mechanics" are very well tuned to avoid various degenerate states I see forums in. Anything compartmentalized into subforums is pretty much a no go for me because the thinking in each subforum gets so constrained.

I agree with GP that the startup stuff can get a bit dominating at times though; a way to filter out say all submissions regarding YC startups would be the icing on the cake, for what has quickly become my favourite online forum.