Why isn't this site as successful as Craigslist?
pg: I wouldn't suggests a frontal attack.  I'd suggest initially focusing either on a type of ad Craigslist is ill-suited for, or a technology that would be very useful in classified ads but which Craigslist ignores because they never change the site.  Use that to get a toe-hold, then gradually expand.  Ideally the new approach would seem as different from Craigslist as Craigslist does from classified ads in newspapers.