Gregory's ServerCraigslist's UI isn't in need of any updates but I do wish they could crack down on spam. Every job search has Lyft ads that spam every keyword under the sun, and they post in every location multiple times a day.
The prevalence of scams would be a much harder issue to solve, since the barrier to entry for the site is so low and there's no identification process. But I *almost* prefer it that way; it's important to be able to spot scams on your own.