I do believe that DirectX is one of the greatest achievements of Microsoft.
Not only this SDK provided, from the very beginning, a clean and efficient way to develop high performance graphic applications for Windows (anybody remembers WinG?), it is also an bright demonstration of Redmond's giant mastery in the art of backwards compatibility.
Every time I put my hand on a new version of Windows, I run PackAttack, a basic DirectDraw 2D action game I wrote several years ago. The last time I compiled it was in December 2000. If memory serves, it was compiled for DirectX 3. I am flabbergasted to see this game still working with DirectX 9 on Windows XP.
Anybody with Vista out there who would dare trying to run PackAttack and see if the miracle still happens, despite the fact Microsoft is supposed to have lost the API war?