Er... you do know that it is because Carmack programmed a special code path for NV30+ ? ATI cards just use the standard OpenGL ARB2 path. And another reason it "does so much better" is because it is essentially a game using DX7/DX8-level of technology.
Valve = ATI / MS (DX 9) sell out.
Valve wanted to put advanced effects in their game. OpenGL didn't support these effects. DX9 did.
And DX rendering engines can run as fast as OpenGL ones, it's just a matter of how good the programmers are. It's just an API you know. This comment was edited on Sep 16, 08:29.