I want blogs to look like Usenet, so that best affordance for a response is another blog post, not a comment. Comments are evil.

There's enough structure in blogs (blogrolls, tags, date, author, backlinks) --- especially if you mine comments --- to do take a stab at this.

I'm interested to hear why you think comments are evil.

Would there be any way to make that discussion-like structure accept input from anonymous, infrequent, or lazy users? Setting up a blog just to reply might leave out some decent responses.

They're second-class outlets. A blog author can say whatever they want; their posts are first-class elements in Google's corpus; the medium encourages stand-alone writing instead of meaningless squibs.

The original vision of the "blogosphere" rejected comments. "Write a blog post, instead." Here's an old, well-known example:

http://www.markbernstein.org/Apr0401/Whyblogcommentsarebad.h...

The problem right now is that the affordances favor comments over posts. The first step is to adjust them; it just so happens that by making a threaded blog-group reader, you're also providing a lot of value to people who don't care that comments are evil, but do find it hard to follow 10 different blogs talking about the same thing.