On-topic guideline essentially allows everything, rendering off-topic guideline moot. Could have been avoided by defining "a good hacker" but that's missing as well :)
Given that it's his site, his forum and his rules, I've always assumed "anything that good hackers would find interesting" loosely correlated to "anything Paul Graham would find interesting."
That said, "good hacker" as a qualifier is as frustratingly elitist as it is maddeningly vague.