Ripped from a blog that ripped it from another blog:

TPM:
But if you look at Palin's interview trainwrecks things always got bad on the follow up–when the interviewer (Gibson or Couric) pressed her on the nebulous answer for some specifics, which she couldn't provide. That's the difference.

Daily Howler:
Duh. In part, Palin did better because Gwen Ifill asked no follow-up questions.