Is it the interactivity, the commenting, the archiving? Perhaps the cloud tags, the permalinks or the reverse-chronological order?
In my mind, all of these elements are not only optional but also secondary to the most critical aspect of a blog: RSS.
Most of you know what RSS means: real simple syndication. This is code that is embedded into a web page and not apparent on the page but is picked up by search engines and RSS readers when a page has been updated with new content.
I think that folks mistakenly believe that blogging is a style of writing, a design template or a body of work.
I see a blog is a technological invention that ensures that search engines have the most current and most popular information. In fact, it is well known that search engines give preferential treatment to blog content over regular web pages.
Ergo, all remaining elements of a blog are just window dressing.