Because there are other sites that already serve as sources of general news. I come here looking specifically for "hacker news".
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.
Off-Topic: Most stories about politics, or crime, or sports, unless they're evidence of some interesting new phenomenon. Videos of pratfalls or disasters, or cute animal pictures. If they'd cover it on TV news, it's probably off-topic.
So yes, things that gratify one's intellectual curiosity. But typically if it suggests a new trend.
By way of example (of what I think not what the rules are) YOU may find articles on high frequency trading to be intellectually satisfying. The majority of HN may not. A few articles as the trend emerges will be generally interesting to the whole crowd here as an emerging trend. But after that it is probably a very specific area of interest that is much less of what we all want our front page filled with.