I love Digg for a lot of reasons, but the problem is that stories and comments are not dug or buried for their merit, people do so based on whether or not they agree with it. For the comment system in particular, that makes it broken.
The new comment system demonstrates this very well. While I think the new system is pretty cool, some people hate it. If you read the comments on that story, the people who do like the new system just get dug down anyway, even if they have an opinion with merit.
There are a great many things to think about though in terms of what they've done here. Daniel Burka has a blog post explaining many of their design decisions. I think that for the most part he makes a great many compelling arguments. There's a constant battle in my mind about how to build a forum discussion. In real life, even a large group of people talk in turn, one after another. A linear style thread, like those found on most forum apps, is recorded this way but it's not real time the way a human conversation is. To compensate, that means you have to quote previous posts, and far too many people never bother to trim those quotes so you end up with a ton of repeat or distractionary data.
On the other hand, you can do the Usenet-style true threading, but the problem is that a conversation can then splinter off into a thousand different directions, which is, frankly even harder to follow. It can lead to many posts spread out by the same user too, often saying the same thing.
So I think that the linear threading is still better in most ways, but you still have that problem with quoting. I'm not sure how you solve that problem.
The thing I do like is the AJAX loading of posts. The theory of it at least is interesting, in that you have less database activity, less initially transmitted HTML and a more manageable page for the browser to render. I'd be very curious to know what the measurable impact of this is.
The online discussion has not really evolved all that much in ten years. I wonder now what imagination will lead us to.