I'd like to see the blog round out the P-SE community a bit, rather than merely amplifying something we've already got. The "best question" bit, for instance, could feed content every once in a while, but I think the blog has the potential to address a few things that the rest of P-SE, quite definitively and intentionally, is not.
First of all, the blog itself doesn't have to have one author exclusively, but each post is the work of a single author. This gives the author a platform to bite off a chunk of topic (of his choosing) and cover it in as much detail as he sees fit without worrying about whether he'll be voted up or down.
Next, a blog lends itself well to covering large topics as a series of posts. That's something we've found difficult to do by tying questions to one another. In this case, the author controls the macro-organization of the topic and the timing of posts.
Finally, the blog should absolutely be free -- encouraged, even -- to cover the subjective aspects of photography that are avoided or approached with extreme care in the main site. Go ahead and tell me why Canon blows chunks or that you think the exposure triangle is The Great Satan. Stir the pot a little. Just make it interesting, and make sure to leave comments open. These posts can function as lightning rods for controversial topics. Remember, the goal of the blog is to be interesting -- not to please everyone. I want to hear opinions!