Mike makes a great point about how well-received Palin's speech was among liberal elites. Lots of them deemed Palin's homespun shtick authentic and assumed it was what actual homespun people want. Fortunately, I think it's just another example of the elites being out of touch.
My sense is that, while most PTA mothers don't want politicians bashing them, they don't see PTA membership as a qualification for the presidency. By playing up her ordinary mother bona fides, Palin demonstrated that she's a regular person. But regular people don't necessarily want a regular person in the White House.
About a year ago, I tried to explain why downscale voters often shun authentic populists for phony populists like Fred Thompson and George W. Bush. I argued that a lot of downscale voters know Thompson and Bush are rich and that their populist shtick is phony, but find it highly appealing nonetheless. They have nothing against rich people--many aspire to emulate them. They just prefer their rich people folksy and unrefined rather than haughty and pretentious.
I think you can tell a similar story about presidential qualifications: Downscale voters want highly credentialed people who act down-home and small-time, not uncredentialed people who really are down-home and small-time. I suspect Palin is too close to the latter. The McCain campaign would have been much better off highlighting her credentials before showcasing her folksiness.