I always liked hacker news discussions. The root post is mostly a link, but can also be a regular post. The discussions are threaded. Posts in a thread are sorted by up votes.
It has no social features. But because I use it a lot and I see a lot of quality posts on it, I really wished they have some social feature. For example, 1. allow me to subscribe to a thread 2. allow me to follow a user. so any thread that a particular user commented or upvoted will be of interest to me. 3. they don't even notify me for responses on my comments. so there is nothing social. but I like the discussion interface.