I actually love the design of Craigslist. it works well enough for you to find what you're looking for (if it exists), but it does not invite you to linger afterwards.

I have to be honest, the 'craigslist of web forums' is the entire appeal for me. It's simple, fast, low clutter and in my opinion benefits from great moderation and a general involvement from the community to avoid treating it like a gutter. I could be suffering from limited imagination, but I'm not really seeing major opportunities for improvement. Change? Yes, for sure, lots of ways to change the site but I don't know how they would result in an outcome of improved community or experience.