Perhaps the difference is, as Felix says, that readers are being persuaded to pay but are not absolutely compelled to pay if they want to read everything for free.
5. Bonus reason why it’s all less significant than at first glance.
To a certain extent, Sullivan and his crew are, if not sui generis, an anomaly on the Web—one of only a handful of established bloggers able to draw what amounts to a mass audience, month after month, year after year.
In The Myth of Digital Democracy, published in 2009, Matthew Hindman assembles the data to show that, for a number of technical and cultural reasons, a small number of bloggers—and Sullivan was one of those singled out—dominate traffic heading to politically oriented sites. A lot of traffic goes to a few sites, while the vast majority gets very little. Hindman calls this the “missing middle.”
That’s why Sullivan’s experiment, even if it succeeds—and I wish him the best of luck—may not be as much of a bellwether as you might think, or hope.