Interesting series of posts and comments, upon which I will tack this. See: danah boyd's categorizing blogs (also posted with more comments as Defining and Categorizing Weblogs), Liz Lawley's defining blogs, Shelley Powers' Lines of Communication, and Joi Ito's Comment spam and its social equivalent.

When I was a kid we used to go out and "play baseball". This meant 3-4 kids with a bat and a ball and some mits, rather than two teams of nine players, a diamond, umpires, etc. Technically, you couldn't categorize the game we played as the game of baseball, though you could perhaps say it was inspired by the game of baseball. No matter to us, we were playing baseball.

This is the model I use when I think about identifying what is a blog. Web logs (and the earliest blogs) were something that inspired the blogs of today. And, out of that inspiration, today's blogs exist in many formats while tomorrow's will exist in many more, including ones with fewer and fewer features in common with the earliest web logs.

(At this point, at least) what a blog is matters little when you instead look for people who are blogging. Whatever format the blogs take—even if they are unrecognizable by yesterday's blog definitions, we will continue to find people behind these "blogs" who will be blogging.

Ultimately, blogs aren't the phenomenon: blogging is. As I said in July, blogging matters (more than blogs).

I do think there is something to consider though when regarding the affect of comments on a blog: is this actually a discussion forum / online community site and not a blog? But, I think the question is not whether or not a blog can have a discussion system, but how a blog relates to the space where the discussion occurs.

In particular, comments and trackbacks on blog posts represent an attempt to concentrate a discussion within a single space / context that is publicly accessible. I think what needs to be explored about the relationship between blogging and discussion is not whether or not they can co-exist but how or not to concentrate them together within a space or context.

Spam in general is about exploiting a concentration of attention within a space (e.g., an email or web address). Where you look is where people go to be seen by you!

What happens when you look / go to the same place all the time and everyone knows it? You have to deal with moderation and games in confined spaces (as I described it in October).

Ultimately, I think address / context portability is an important feature in the future of all of this: your blog and its discussions and the identities used to go through barriers of entry will be able to happen in many different web spaces while retaining cohesion. This will distribute and diversify the blog / discussion spaces making them both harder to classify (as nouns) and harder to spam ;-)