It's not pretty, but that's about the least important element of web-design.
It's very easy for people to understand how things are organized. Cities are organized together, categories are organized together. All of the pages are text-based so pages load fast. Various functions on the page are explained with helpful text explaining what's going on when you try and post or respond to a post. Etc.
Their UI can be improved with a little more padding in places and a little more focus on quality typography, but it's remarkable that such a large site has maintained such a quality design in the face of enormous pressure to change.