A great blog post - as opposed to a "check this out" post - is succinct, non-rambling, fact-filled, gracefully-written, minimally self-referential, and pulls together disparate themes and unusual facts to shed fresh light on a subject, or to make a random subject interesting. That is to say, a great blog post is a good brief essay, ideally well-salted with links. When I post, I rarely have take the time that is required to be brief.
Here's a good example of a fine blog post by Never Yet Melted about deer coursing in Scotland.