Reddit comes close. I don't know that there can really be a 'canonical' discussion site, any more than there can be a canonical Q&A site (Stack Overflow is not that, given the existence of Quora, Yahoo Answers, etc. serving other large niches of Q&A).
Slate.co looks like it could evolve into a viable contender in this space. I have a few qualms about the way questions are structured, but the site is pretty and nice enough.