The emergence of the practitioner-blogger has the highest potential significance for arts journalism. Many, perhaps most, of the greatest critics in history — George Bernard Shaw, Virgil Thomson, Edwin Denby and Fairfield Porter come immediately to mind — were also practicing artists. But with the growing tendency of mainstream-media journalists to think of themselves as members of an academically credentialed profession, the practitioner-critic has lately become a comparative rarity in the American print media. Not so on the Web, which is one of the reasons why readers in search of stimulating commentary on the arts are going online to find it.