@pg I think a great one would be "b2b customer acquisition". This was one of those things that we realized was CRAZY HARD and (anecdotally) I've heard a few other YC startups who have run into this wall. Stuff like inside vs. outside sales, lead generation techniques, hiring salesfolks, adwords, etc.
Of course this stuff only applies to maybe 25% of YC companies out of the gates (tho eventually, even the consumer plays have to hire ad-sales guys).