I'd prefer to post comments without registering. If i have to register, I'd better not post comments.
It depends - if you're in my situation where your blog gets lots of one-time visitors then you probably don't want to make it too complicated for them to comment. If you get lots of return visitors then registration is an option.
I don't think Blogs should ever make readers register, it's a put-off people don't want to spend time registering they just want to have their say then be on there way As mentioned already, allow anyone to comment just make sure you have askimet or another spam deterrent installed.