One thing that I only thought of recently is how on youtube you can "discover" videos via the "related videos" side bar. I'm not aware of anything like this in blogging sites.
Imagine the same thing for a blog. Actually I don't like the word blog, but I'll use it for lack of a better term.
Imagine you're reading a blog post, and on the side you have similar/related blog posts from many different other people.
If you're a reader, this is good because you can discover more content easily. If you're a writer, this is good because your content can be discovered more easily.
Something similar is happening on facebook. People make facebook pages and write their thoughts there, it helps them get more exposure, even though the facebook UI is not optimized for these kinds of things.
I like the dynamics of the non-linear "discussion" that take place on youtube, and I think there's value in that.