I guess the McCain campaign is going for Palin as an antidote to the age factor? Except that already came off the table to some degree with the choice of Biden, who is in his 60s. It does give McCain/Palin the opportunity to appeal to women, but I don't see that many women who were on the fence going for McCain/Palin just because she's a woman. I could be wrong, I guess.
The thing that immediately struck me, though, is that this is now the all-under-investigation ticket. McCain's history (Keating 5) combined with Palin's current struggles (under investigation by the Republican-controlled state legislature) makes for an easy line of attack.
Most of all, though, she just does not seem presidential. And she's one 72-year-old-four-time-cancer-survivor-heartbeat away from the presidency if McCain wins.