What makes a blog successful?

I've just taken a look at a report by Northeastern University and Backbone Media - Blogging Success Study. With 3 sections and 5 appendices it certainly delivers the thud factor. They interviewed a group of self selected 'successful' corporate bloggers who had been blogging for more than 12 months.

It's a got some interesting things in it. Not least: "While the selection of participants was, ... subjective ... (without the resources to determine the most successful bloggers on the Web) ... "

How much resource does it take to assess the most successful bloggers on the web? I can't imagine that it would take longer than a short morning to assess the 'success' of their interviewees against each other and a benchmark like Beppe Grillo or Steve Rubel. But perhaps the techniques that I would consider using wouldn't be regarded as sufficiently rigorous.