Network effects are huge, and especially tough to overcome when the price of listings on Craigslist is so low, but I think there's another effect at work: namely, their minimalism actually makes the site more useful for their users than a more structured listing model would be.
By storing only very minimal posting metadata, and eschewing an API that would allow mash-ups and alternate browsing interfaces, Craigslist offers the same (often overlooked) advantage that old-school classified ads do: namely, they more or less force people to read a lot of the ads in order to have any chance of finding something.
That's better for sellers, because it increases the chances their ad will be seen, and in many cases it benefits buyers, too, since they may stumble across ads that would never have shown up in a more sophisticated search result set.
By storing only very minimal posting metadata, and eschewing an API that would allow mash-ups and alternate browsing interfaces, Craigslist offers the same (often overlooked) advantage that old-school classified ads do: namely, they more or less force people to read a lot of the ads in order to have any chance of finding something.
That's better for sellers, because it increases the chances their ad will be seen, and in many cases it benefits buyers, too, since they may stumble across ads that would never have shown up in a more sophisticated search result set.