Earlier quoted context omitted.
The funny thing is that given the space allocated to cover all those scenarios, the applicant ends up in a situation very similar to game theory (a post yesterday on HN pops to mind). You, as the applicant, have to figure out what PG will consider non-obvious so you cover those, but keeping in mind that if there is anything obvious that PG would have thought you should cover, then you should cover those as well, but…
Actually there is a standard for non-obviousness: what's not obvious to your competitors. We have a question specifically about that. What do you understand about your business that other companies in it just don't get? The amusing thing is, this question turned out to be an instance of itself. Many of the YC clones who copied our application form cut this one, because its purpose wasn't obvious.
I now see it's also about noticing non-obvious things, as a way to evaluate the founders; and that this better fits YC's philosophy of funding people not products.