Set the bloggy flags at half-mast, for Dave and Greta are shutting down Cognitive Daily. OK, maybe three-quarter-mast, because they're doing it of their own free will, but still, they'll be missed.
And now I need to find a new example of an extremely successful blog that is always and unfailingly about science, with no excursions into politics or media criticism. Suggestions welcome in the comments.
Terry Tao's blog.
Terry Tao's blog is a slightly different thing than Cognitive Daily was, though. It's mostly class notes and formal derivations, for people who are mathematicians or at least serious math students.
Cog Daily was more of a popular-audience science blog-- good, solid articles about current results in science, written for an audience of non-scientists. I was really impressed by how successful they managed to be at that while never succumbing to the temptation to throw up a few political posts or cat pictures to drive traffic.
What about "Not exactly Rocket Science"? I guess Ed does a little media criticism, but not much. There's also BPS Research Digest.