At one time, Gates seemed to have fearsome powers. The "BillG Review" was an event to be dreaded, but most of all an event for which to be prepared. Joel Spolsky gave an account of his first BillG review. That review sounded tame compared with this one with the Gates classic line of "I can't believe that anyone with any f-ing IQ is working on this problem."
Joel Spolsky's take on the de-evolution of Microsoft provides a clue as to why Gates lost his lust for Microsoft:
Steve [Ballmer] took over the CEO role on the theory that this would allow Bill to spend more time doing what he does best, running the software development organization, but that didn't seem to fix endemic problems caused by those 11 layers of management, a culture of perpetual, permanent meetings, a stubborn insistence on creating every possible product no matter what ... and a couple of decades of sloppy, rapid hiring has ensured that the brainpower of the median Microsoft employee has gone way down.
When Bill focuses on his charitable foundation, he'll be back at the helm of a small but well-capitalized organization. Unlike Microsoft, though, it's not one that needs to answer to stockholders and I suspect he'll keep the organization tight and responsive. I have to think that is one of the primary reasons he's leaving Microsoft. He wants to work at an organization where he can make decisions and there is at least some hope that the company can make them happen. Those days are gone for Bill Gates and Microsoft.