Bush, Biden, Gore, Kerry, Obama, edwards = all very smart men. Granted, some of them were lazy and not exactly thirsty for knolwledge, but all have the ability to understand higher level concepts. Hell, they all started or finished grad school - and they all attended serious schools.
Palin limping out of 5 community colleges with a journalism BA degree, and a 2.2 GPA in high school, is simply not on their level. We all know people like that, working at Walmart as a customer service manager. Good enough to get by, but certainly not on the higher level of the others.
When Bush, Kerry, Edwards, and others aren't in office, they're instantly employed at think tanks, where their mental ability and views and experience are put to good use investing and shaping policy.
Palin's gifts simply aren't in that mental area. She's a good entertainer. She's good at making speeches to an angry 20% of the population. She's good at reading 4 answers off a hannity teleprompter, then stumbling thru 2 follow up questions on her own. She'll make a great living - but she'll never be a scholar, never be a policy shaper. Even in office, she'd simply just cheerlead legislation that "her team" gives her.