If Paul Graham's essays are an ongoing course in entrepreneurship, then this video is the Cliffs Notes.  One thing I've learned from reading him over the years is that it's OK, or maybe even preferable, to acknowledge and face up to the things you don't know.  The 20:00-22:00 marks of this video is a parade of I don't knows and Paul is almost proud to admit that he doesn't know.  "Cisco?!  I don't know - IBM!"