There are plenty of usability improvements to the Craigslist UI that have nothing to do with adding "annoying banner ads" or a login screen. For example, suppose you're looking for apartments on Craigslist. Suppose I want to find all the apartments near a certain location. Craigslist lets me search/filter by city and by neighborhood, but that's about as granular as it goes: there's no way to say "show me apartments near address X", as far as I know.

The reflexive response to comments about Craigslist is always "you don't get the Craiglist ethos", but IMHO they aren't exactly the epitomy of good UI design, either.