This is unexpected. Mayer did a lot at Google, involved heavily in the UI design of Maps, GMail and even Google search, but I fear Yahoo is already dead as so many employees have already fled the company. However, there is no doubt Marissa is the best bet to turn around Yahoo.
Current executives have shown they have no clue what to do. Marissa will, at least, have a fresh vision, unencumbered by any embedded boardroom bureaucracy.