I don't have stats, but I know that when I visit a site that uses Disqus I'm more likely to comment. I hate filling in the personal information form repeatedly, and this way, my comments always link back to my site, and I can keep track of all my comments in one place.
Actually for me more the most important factor to comment is valuable or interesting information for me. When i see it, i comment and even try to share with others. The second factor is a blogger, who has earned good reputation and write precious info, share His knowledge. I think whether the site uses Disqus or no, there's no difference
Disqus versus requiring people to make accounts on your site. Disqus will win out.
Versus anonymous comments I'm not sure, but anonymous comments usually require capture or something annoying.
It is a great system. We are using it on a site and we are now receiving many more comments. Spam isn't an issue either. We recommend it to anyone who needs a commenting system.
It's true for me because I like the design of disqus. Any other commentating system with a similar design would encourage me as well.