When an article is shared on Google+, all the shares are shown as comments on the blog post. All the comments on that share are shown as well. You can choose who your comments are visible to, and even post them on Google+ as well. Because that's what these comments are. They're Google+ posts.
When viewing other comments, it's no different than Google+. You can +1, reply, and more. Plus, you can also sort the comments and even show only ones from those in your circles.
This is the Google+ integration we've been looking for. The borders between sites are beginning to fade in the Google world, and with Google I/O less than 1 month away this can't be anything close to the end.