Despite her inability to fabricate information she doesn't have, Palin is rather adept at political BS. She has a demonstrated ability to effectively deliver pre-packaged talking points, be they off a teleprompter or from memory. The challenge was to ensure that Palin would not be knocked off her talking points. They had to turn the debate into a speech.
They had a few tools at their disposal. First, Joe Biden, Palin's debate opponent, is a notorious "gaffe machine." The Obama camp had a comfortable lead coming into the debate and wasn't enamored of the possibility of Biden running his mouth a bit too long in the highest profile event he's likely to headline.
Second, (and I'm speculating here) they had some dirt on moderator, Gwen Ifill. The fact that Ifill has been writing a book on the african-american political experience, including a chapter on Barrack Obama, was well known before the McCain people agreed to the debate. They could've brought it up at the time but, by agreeing to Ifill as host, they had something to tar the moderator with should things not go their way.
Now the stage was set and it was time to pick the format of the debate: 90-second responses with no follow-up. Ifill was effectively neutralized as the follow-up, the bain of Sarah Palin's existence, is off the table.
Come debate night, the outcome was predictable. Whenever Sarah Palin was asked a question outside of her prepared talking-points, she simply didn't answer it, choosing instead to rattle off a prepared screed on an unrelated topic. Thus, we got a lot of unasked-for fluff about energy policy and whatever the hell she was talking about when asked about her greatest weakness. Even if the format had allowed, Ifill was not in a strong position to insist on any sort of substance from either candidate, especially Palin.
An effectively-moderated, actual debate might have turned out substantially differently but neither campaign wanted to take that risk. Whoever won the debate, we lost.