People complain all the time that they can't be bothered leaving comments on blogs because they need to sign up to do so. I don't have links to substantiate this but do a search online and you'll find it's a very active topic of conversation. The actual comment policy itself is also a much talked about topic. People have differing views and each has its pros and cons. For example, allowing only registered users to comment ensures that each comment is "tied to an identity". I think this is important. If you're going to leave an opinion, at least have the guts to tie it to your "name brand". Problem is, it presents a barrier and many can't be bothered signing up just to leave a comment. Only people with compelling reasons to leave a comment will bother. On the other hand, if you leave commenting open to all, you subject yourself to spam and anonymous comments. Spam comments are obviously bad but I'm not a fan of anonymous comments either. Having an opinion is great, but it is a little cowardly not to leave your name.
Up until now, I've kept the "only registered users can comment" option enabled in my blog settings to avoid comment spam and tie comments to people, but I've decided to conduct an experiment and see if disabling it will actually remove the barrier and encourage comments. I know I'm opening myself up to spam comments, but there are settings that can be made to alleviate that issue like forcing all comments to be moderated (by me) before being made "live". I can also change the settings to force a commenter to enter a CAPTCHA upon posting a comment thus ensuring they are human (hopefully) and will mean no automated spam left by Bots.
For now, I've left those settings turned off. Comments are open to everyone. No moderation by me. No CAPTCHA to fill in.