I would enjoy an HN-like site with a greater focus on algorithms and other more complex material.

HN sometimes gets these posts on things like closures, functional programming, monads, y combinators, etc. A lot of these are just posts from Wikipedia or such, but they're interesting and generate a lot of discussion.

Similarly, I enjoy the more physics and chemistry oriented tech articles.

To summarize: I'd love an HN for theoretical CS, physics, chemistry, etc, more focused on the actual sciences than on marketing, sales, deployment, etc. I'd be there in an instant.