A columnist can ignore or duck a subject less noticeably than a blogger committing thoughts to pixels several times a day. A reporter can wait—must wait—until every source has confirmed. A novelist can spend months or years before committing words to the world. For bloggers, the deadline is always now. Blogging is therefore to writing what extreme sports are to athletics: more free-form, more accident-prone, less formal, more alive. It is, in many ways, writing out loud.
I would add that in a technical area like science blogging, being 'accident-prone' takes on a whole new level of meaning. If political blogging is, as Sullivan says, a big jump from news magazine commentary, the chasm between science blogging and technical journal writing is daunting.