One problem we foresaw with that approach is that it becomes hard not to have duplicate discussions or missing chunks of information. In the end we did not do anything with the idea but it was certainly interesting.
In the past, people like to read newspaper in the streets. Now, we read news from HN. If there is no comment box any more, no one can try to be a superstar in such a prominent place. Then we can just upvote and downvote a news, then talk about it somewhere else. And HN can let the submitter recevie links about discussion from others and show them under the post. Or just receive any links, then the submitter can place it at another positon.
What if it were geo-local? When you post an article or text post everyone sees it but you only see comments from people within 100km of your location.
You definitely can't just do everyone within a given radius from you because a person on one side of you won't be able to communicate with someone on the other and that's incredibly confusing. You could split the buckets up by location, but some people would have to put up with living on the border (well, just like a country or state).
There is one startup spun out of Stanford that is doing MOOCs by emphasizing the small group. At the beginning of the course, you can either create your own group and recruit others or join a group (generally 4-8 people). Those who do neither after the first week or so of class are randomly assigned into groups with others who were also not proactive about joining a group. It is easier to do discussions and group projects when there is a permanent group of 4-6 other students, and some of the courses have components where the group will submit work together and the rest of the class can see it.
I imagine tuning such a system would be very difficult though, and you don't want users stuck in a bucket that they don't feel fits them.