Rice is a smart, accomplished and ambitious upwardly mobile academic, far from brilliant, who landed a job as policy adviser to the President and moved from there to Cabinet status. Very few people have the peculiar mix of brains, talent and career instincts that are required to follow that particular path to the top.
IHMO, what she lacks to be President is the ability to schmooze with, understand and manipulate the politicians, particularly Congress, in a leadership role. She's not one of them - - she's never run for public office and failing that, she's never held a top-level military command. Either would have done it for her, (although possibly as she's a woman, the military command, largely symbolic anyway, might still have failed to produce the desired effect) or at least been a step in the right direction, but the lack of both is fatal to her chances.