If you take a moment to browse around the news section of the major three search engines you will find that Google rarely features an article highly if an open comment system is available for that article. Of course Google will include such a page in their link of "other" sites discussing the topic but you will likely not find even one article that doesn't force registration to that site or has no comment section at all.
That is in stark contrast to both Yahoo and Bing news sections that feature both equally on any given topic.
Come to think of it, if you look at Google's own blog search you'd expect to find many blogs with commenting sections however even the sites featured there do not often have articles with open comment sections, most in fact aren't even blogs(e.g forums, news, etc...)
Why is that? Is it Google policy to avoid featuring sites with open discussion platforms that don't require doing more than listing a name and email to post? Would it be good policy to pay attention to this fact when choosing a discussion platform on our sites?