It might seem obvious, but it’s worth saying: blogs are about communication. Communication isn’t the same thing as dissemination/syndication, as it implies that readers can participate in a post with critiques, questions, and additional information. Reader participation makes a blog more than simple announcements, it elevates a blog from a simple homepage1 to being a bazaar of ideas.
Participation can take many forms: comments being the most immediate (since the reader can easily browse them when reading the article); but automatic backlinking works too (see pingbacks). The irony is that adding that kind of a system to a blog makes it intrinsically more interesting, both to the reader and the writer.
Rant trigger: some guy presents an idea without any mechanism of receiving feedback. Honourable mention: dmo asks for tips on his blog, without providing a mechanism for readers to comment.