Wait, so OpenGL 2.1 supports WDDM, Hardware reset & failure recovery of DX 10 & XNA Gaming Platform too? Wow, where did these come from then
Here is the thing, right now OpenGL 2.1 & DirectX 10 provides almost equal features. Both support unified pipelines, both support stream processors. However the difference is that DirectX is already optimised for Windows Vista, which saves a lot of developer work to optimize the code & engine of the game for Windows. OpenGL on the other hand, despite of being very powerful, needs to be optimized according to the OS as it comes without any optimization out of the box. It provides similar graphics quality but not the same lavel of efficiency when running on the most used Computer gaming platform out there, Windows.
Besides, OpenGL 2.1 doesn't support the Microsoft XNA Platform for game developement. Using XNA, a game developer can make a game in DirectX 10 for Windows Vista & easily port it to XBOX 360, in most cases just remaping of the controller & CPU specific optimsations are required.
Although, if Khronos group can provide OpenGL for Vista in such manner that Vista Aero can rely compleately on it, then maybe we can get Aero Running on OpenGL 1.5 based cards like geforce 4 & FX faster then DX 10.