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

They're competing with a specific part of Craigslist, namely short-term accommodations.

Other companies are competing with other parts of Craigslist (e.g. StubHub)

Funny, because they seem to get quite a few referrals from craigslist.

Wikipedia gets quite a few referrals from Google. Are they direct competitors?

That depends on your view of Knol.