You'll likely see companies successfully spin off of "parts" of Craigslist, which are being neglected. The major issue with Craigslist is that it's not organized well. It's extremely chaotic, requiring users to spend lots of time searching the site, learning where stuff is, and memorizing its ebbs and flows. It's this giant database of useful information which is screaming for somebody to make it simpler to use. CL prevents this with their lack of 3rd party developer support, so different databases will pop up and slowly chip away at them until they either open their API's, or die off.

Ditto that, I actually emailed them to get their 'API key' to write an iPhone app for the site. They told me they only got half way through writing the API and then decided to give up and keep people interacting to their site through their mobile browser. :S