James Fallows has a must-read post on why Ms. Palin's cluelessness about the Bush Doctrine "implies a disqualifying lack of preparation for the job":
Each of us has areas we care about, and areas we don't. If we are interested in a topic, we follow its development over the years. And because we have followed its development, we're able to talk and think about it in a "rounded" way. We can say: Most people think X, but I really think Y. Or: most people used to think P, but now they think Q. Or: the point most people miss is Z. Or: the question I'd really like to hear answered is A.
Here's the most obvious example in daily life: Sports Talk radio.
Mention a name or theme -- Brett Favre, the Patriots under Belichick, Lance Armstrong's comeback, Venus and Serena -- and anyone who cares about sports can have a very sophisticated discussion about the ins and outs and myth and realities and arguments and rebuttals.
People who don't like sports can't do that. It's not so much that they can't identify the names -- they've heard of Armstrong -- but they've never bothered to follow the flow of debate. I like sports -- and politics and tech and other topics -- so I like joining these debates. On a wide range of other topics -- fashion, antique furniture, the world of restaurants and fine dining, or (blush) opera -- I have not been interested enough to learn anything I can add to the discussion. So I embarrass myself if I have to express a view.
What Sarah Palin revealed is that she has not been interested enough in world affairs to become minimally conversant with the issues. Many people in our great land might have difficulty defining the "Bush Doctrine" exactly. But not to recognize the name, as obviously was the case for Palin, indicates not a failure of last-minute cramming but a lack of attention to any foreign-policy discussion whatsoever in the last seven years.
Fallows has more on this, so do read his entire post.