When a company's culture and identity is so strongly bound up with that of its leader, it takes immense strength of character for that leader to plan a robust succession and choose the right moment to go. I take my hat off to Bill Gates for both his careful planning and his timing.
I think he has timed his departure to perfection. As others have remarked, this is truly the end of an era. The heyday of software is over. Software will still be important, but its role is now fading out of sight, into the infrastructure. Gates' choice of successors to guide Microsoft's continuing development strategy acknowledges the shift of focus towards selling not software itself but the functionality it enables, delivered across networks.
(For those readers who haven't been keeping up, here's a reminder from previous posts of some of the straws blowing in those winds of change:)