The issue that most of these services seems to be trying to mediate is "truthiness" or "authority".  Assigning some sort of integer to the value that a commenter typically brings to a discussion.  The first such plugin that I tried was Buddycards, which were relatively non-invasive as far as layouts go.  The problem being that not enough people use the 30boxes service, or were willing to create another profile just to have an identity to map to their truthiness.

The next one I tried (that I remember) was SezWho.  All of my comments were imported there, so that users can claim their comments here, to obtain better karma elsewhere.  Disqus followed later this year, you can claim your comments at archgfx.disqus.com.   The problem with both of these is that the company's entire business revolves around comment authority, which is inherently tarnished by a profit motive.  If we can't trust the company, how can we map real trustworthiness to their scale of authority?

Centralized anti-spam services play largely the same game, and indeed disqus appropriately cuts out Akismet, Defensio, or any local spam prevention measures.  Ultimately, anti-spam services don't try (yet) try to merge disparate communities.  That's the biggest problem I see with comment authority - The authority I would have on a music blog is different from the authority I might have on a tech blog.  The comments I might leave on a feminist blog would be little more than conjecture or anecdote.

Ultimately, the reason that I, and most people choose to run our own blogging app, instead of using a hosted service like wordpress.com or blogspot, is that we don't want to be dependant on the standards and defaults of a larger, authoritarian group.  By removing the ability to customize, and normalizing the authority of commenters, commenting services like disqus take away the things I like about having my own blog, even while they attempt to mitigate the unpleasant elements of spam and trolls.  Anonymity and decentralization are linchpins in the present nature of the internet.

I haven't bothered installing Intense Debate or JS-Kit yet, because each new plugin from a non-reputable source is another security risk, as I allow it to rummage through my database for comment information, and email addresses of people who wordpress assured wouldn't be shared.