On-Topic: Anything that good hackers would find interesting. That includes more than hacking and startups. If you had to reduce it to a sentence, the answer might be: anything that gratifies one's intellectual curiosity.
While I agree with you, I think it's due to submissions such as this that end up leading to things like the recent submission "Solving the Hacker News Problem" http://news.ycombinator.com/item?id=2252152
http://ycombinator.com/newsguidelines.html