Her experience. She forfeits the McCain campaign's attack that Barack Obama is too inexperienced (more on this in a moment). She was runner-up in a Miss Alaska beauty pageant, capitain of the basketball team, and has a bachelor's in journalism from the University of Idaho. After college, she sat on the Wasilla city council for a few years (population: less than 5,500) , lost a primary race for Lieutenant Governor, was a political appointee to the Alaska Oil and Gas Conservation Commission (presumably, with her journalism degree in-hand), quit after a year or so, and ran for governor of Alaska - a position she's held for all of a year and a half.
She's a token woman. Becuase virtually no regular American outside of Alaska has heard of Governor Palin, one questions why she was put on the ticket. Seeing her thin list of accomplishments, it's easy to conclude it's a transparent ploy to siphon women voters and nothing more. It's the ultimate pander. It's patronizing. It's an insult to womens' intelligence. It's as ridiculous as saying, "All black voters will vote for Obama because he's black." African Americans take rightful offense to that statement and women will be equally offended by the parallel implication. Hillary Clinton - who has the necessary credibility among the voters this decision targets - could have a field day with this decision.
John McCain's judgment. This plays nicely into the Obama campaign's criticism of McCain's judgment. "You had your choice of dozens of qualified picks, and you chose her? You believe she has the experience Barack Obama doesn't? This is who you trust to lead a country at war on two different fronts; a country in economic peril; a country facing new challenges every day in rising fuel and food costs, Russian aggression abroad, and the crisis of global warming?" ... which segues immediately into...
John McCain's age. He's 72 years old. Age was a liability before. Are people comfortable with the idea of "President Palin"? I remember an ad from the 1988 campaign similarly questioning a "President Quayle". Prior to that, a very effective ad from the 1968 campaign about "President Agnew". If I could have a nickel over the next few months for every time the traditional media says the words "hearbeat away" ...
Bolstering the Bush-McCain-marriage narrative. Four more years of unqualified appointees. The Bush administration has been sharply criticized for appointing unqualified political/convenient appointees to important public positions (e.g., Harriet Miers, Monica Goodling, Michael "Brownie" Brown, to name a few). This choice shows the same disregard for who's really prepared to handle the important issues facing America. Also, lots of discussion about her relationship to big oil and gas (Oil and Gas Commission appointee, other Alaska lawmakers, outsourcing pipeline construction to Canada, husband with BP - it's like mini-Cheney without the foreign policy experience)
Joe Biden. While he has to be mindful of the media looking for him to "bully" Palin in a debate, this is no contest. Biden's fire and tendency to go off-message is a concern that he may inadvertantly say something "mean", but you have to love the matchup.
Troopergate
This decision is a swing for the fences, all right. The problem for McCain is he just picked up a wiffle bat.
Updated with #8:
"As for that VP talk all the time, Ill tell you, I still cant answer that question until somebody answers for me what is it exactly that the VP does every day?"