One big problem with comments is the isolation. If I leave a comment on a blog, there's little chance that I will return to see responses to it, and I even if I do, it isn't part of my online persona. There's a very asymmetrical relationship between the author to his/her blog, and the commenter to his/her comment. That's why comments don't often match the level that content does.

We are currently experimenting with a solution to this on Hubski. We are allowing some blogs to mirror Hubski discussions of their content on their blog. Here's an example: http://www.theadvancedapes.com/theratchet/2013/1/28/evolutio... I've long seen many bloggers append a post with "discussion on HN" or "discussed on Reddit". We decided to skip the link, and to allow the thread to be embedded. It's not so much a commenting platform as a mirror to the Hubski discussion. So far it seems to be working well, and we have a cue of blogs waiting to be added.

I should mention that users can follow or ignore users, tags, and domains on Hubski, and what they see is a result of what they follow. I don't think this would work on HN or Reddit, as no one would want every post from a given blog to be submitted automatically.