Why he wouldn't: Do you really want to pull one of the world's biggest benefactors back out of malaria research? Also - Gates has never not been involved in Microsoft at a senior level, so he's hardly blameless in some of the less brilliant MS moments.
5. Gabe Newell
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Why he wouldn't: He's already walked away once and may not be seen as an ally from within the company. Valve's setup is pretty distant from the way things are done at Microsoft and leading a company this big is a big ask.
6. Stephen Elop
Why he would be good: Now the front runner by a large amount - given Microsoft's purchase of Nokia's phone division and his return to head up the devices division under Ballmer - Elop represents a safe choice for the Microsoft board.
He's well known and respected within Microsoft and he has big company leadership experience. Elop has also led Microsoft's Office division capably and represents a man who can deal with both software and hardware.
Why he wouldn't: Safe choice does not necessarily mean a good choice - especially when Microsoft faces a real challenge going forward. Does Elop bring enough elan to the role?
Capable, Microsoft through-and-through, and fresh from running a big company that needed to change tack, Elop's appointment would surprise nobody but probably wouldn't thrill too many people either.