Lack of vetting (thank you Cathy), with, among other questions, a cloud of investigation on ethics after a campaign based on ethics.
Lack of experience
13 years of "elected experience" consisting of 1.5 years as Governor of Alaska, 6 years as Mayor of Wasilla AK with a population less than 8,000 during which she fired the police chief for not supporting her political campaign, before that 4 years on the city council of Wasilla.
No doubt she's a "go-getter". No doubt she's ambitious. Nothing wrong with those two things. It's just all the other things that don't add up.