When I go down the list I can only think of a few YC startups that have done well by following these ideas (dropbox, heroku). The leaders in the categories listed are not YC companies, and the big hits that YC did have were in categories not listed (eg. Airbnb).
It's easy to look at the problem with Craigslist and think that to challenge it you should build something which does everything that Craigslist does.
AirBNB is a competitor to Craigslist in that it is helping to take it apart piece by piece. Hopefully other YC companies will take on other categories / pieces
