I don't think Yahoo particularly needs a nice person as CEO right now. Their culture is dysfunctional enough that they probably need a Steve Jobs type, someone with clear opinions who's willing to ruffle a lot of feathers (and make a bunch of people quit). Steve Jobs wasn't really a nice person either.
The biggest risk I see is that there is a huge difference in her role. At Google she stood atop a pyramid of other geniuses, with similar backgrounds and values, and was a filter for their ideas. At Yahoo she's dealing with a culture where engineering is not the highest value, and she's going to have to get off the top perch and descend into the ranks, clearing out the enemies of progress which exist at every level.
It can help you find local maxima, but not the global maximum you are (or should be) looking for.
A big part -- a necessary part -- of the way Jobs changed the culture at Apple was by bringing in a lot of people who had been loyal to him for years at Next and elsewhere. I can't think of any cases where an outsider has parachuted into a large company, by invitation or otherwise, and turned it around by himself/herself.
Can/will Mayer do that? If so, where will the required team of revolutionaries and revanchists come from? If she doesn't (or can't) raid Google, then where will she get the people she will need?
I wouldn't be terribly surprised if one of Marissa's first moves is to fire all the middle management and then promote a bunch of longtime individual contributors into their place.
"Working with Zuck" http://www.facebook.com/note.php?note_id=339013388919
"Every time Zuck looks at a product, it is as if he does so with fresh eyes... He doesn't care what he said yesterday"