Blogs are narratives. Or, at least, the good ones are. Some are unconciously so, the result of one person naturally expressing the narrative of the ideas they're exploring in their own lives. Some are very, very conciously so, with a skilled blogger exploring an idea, or set of ideas, over time, creating a compelling serialised invesitgation of a concept. John Gruber of über-Apple blog Daring Fireball explored this idea a while back in a podcast with Merlin Mann.
This arises naturally as a function of the reverse chronological format that is still pretty much a defining feature of what a blog is, and how it seperates itself from other publishing formats. If you don't have this underlying narrative, what you have is a bunch of individual content publishing in one place. You've just provided atomised content. If you can unify those atoms with a theme, you provide something which is greater than the sum of its parts.
(A few thoughts inspired by some work I'm knee-deep in for those lovely people at Brilliant Noise.)