Well, I can't believe an entire community of entrepreneurs actually beleive this is a design/data use/competition issue and are failing to mention that Craigslist is protecting one of their ONLY sources of income. House postings.
Housing is 1 of the 2 things (job postings in certain cities is the other) that Craigslist charges for. So if Padmapper gets popular, starts listing from it's own website and people go directly to it, bypassing Craigslist, Craigslist takes a Massive hit in revenue.
THIS is a huge weakness. If you want to kill Craigslist start with Housing and Job postings in the cities that Craigslist charges for. Those 2 things ALONE are what are funding all of Craigslist.