I guess that after all it's just like in the rest of the schools.
You have to show that you have great English, that you're not a psycho, that you trully are who you wrote you were in the application and to see that what you wrote as your goals are your real goals. I think that they also invited the ones whom they wanted to hear more about, as far as I understood from my interviewer, he got 5 questions from GSB to ask me and the rest were follow up questions he came up with. They want to know things about us in order to know more and to understand about our fit and our goals better.
That's why a good interview doesn't mean acceptance. We might give good answers, but not what GSB hoped to hear