I think their key advantage is innovation and time. If you are a visionary and a good coder then you'll be able to do the same, otherwise you'll never.
It's not enough. Heck, there are brilliant coders out there, and many of them have vision, but without visibility, you're out.
Craig Newman is a Programmer, not graphics designer. When a programmer design site, it generally become like that. Still its unique for simplicity. It took about a decade for craigslist come to this stage. When they started advertisers didn't have much classified sites for sure which is simple and free. That may be the key for success for craigslist.