Fred Wilson's blog used Disqus, and he engages, but I think most importantly, his blog doesn't have content that interests low quality commenters to hang out there to begin with.
IMO engagement and using a commenting system that is tied to an account helps. However, probably most important is the content itself, and the tenor of the writers on the site. It's no surprise that TechCrunch or Gawker have problems with unruly commenters. They invite that type of discussion.