After reading Marc's rocking response to my images-as-blog-comments post, I realized that I was actually describing a wiki masqerading as a blog post: a small stand-alone wiki initially created by a blogger and 'completed' by contributions from others, a Micro-Wiki, if you will. No one has complained about the ordering of blog comments, so Micro-Wiki is a one-way street too in that contributors are only allowed to add to the post.
Trackback? That's just an entry in a Micro-Wiki. Comments? Same thing. Endorsements? Yes to that too. Thinking about all this leads naturally to structured blog post (fine-grained structure within a blog post) and extensible types of blog post contributions.
Imagine news aggregators that presents you with an updatable list of actions you can take with the selected item (subitem if its structured). Strutured post allows reader to endorse the product being talked about in the post instead of the post itself. Woohoo!