>> "most of the things the YC partners told us we're things that we "knew." Most of the advice was available in one essay or another, but it still felt very, very different to have one of the partners tell us directly what we should be doing."

This is really what will make a difference, that is building a network of mentors that are doing one-on-one mentoring under the same process guided by the same culture. YC should really have a 3rd option, that being to sign-up to become a "certified" mentor.