From my extremely limited knowledge, and mainly from reading, and reading between the line on others people's view. I think Marrissa will be a great COO, but she lacks the vision. Even people who used to work with her ( Eric Schmidt) had said she is the most quantitative CEO in silicon valley, if not the world. And from many other pieces of information she likes to measure everything, from colour, design or human asp…
Remember Fiorina? The same situation of a midlevel absolutely mediocre exec getting lucky at catching a huge wave bringing her straight to the top. Just a female, ie. charming, version of a "Yes"-man.