Those don't come along often, and you can't conjure them up out of sheer will. (Or by yelling and laying off people, as previous Yahoo CEOs learned.)
In a way, Mayer's in a no-lose situation. She's already wealthy and well-regarded, and probably couldn't see a path to the CEO's office at Google. So why not take a swing at the fences with Yahoo?
I wish her well, but I'm not betting on a home run.