Also, my understanding is that on Windows, OpenGL generally runs into more issues with driver bugs than D3D does.
I'm not sure if that's the case anymore, but it definitely has a bad rep on Windows. Initially (throughout XP and maybe some of Vista) OpenGL support on Windows was done by a OpenGL -> DirectX translation layer, so performance was always worse in OpenGL mode unless a game's Direct3D implementation was especially awful. This stopped being the case when NVIDIA started shipping a full OpenGL driver. (I'm not sure when AMD/ATI started shipping theirs)
Initially (Windows 95), OpenGL support was provided directly by the OS. Starting with Windows 98, Microsoft stopped updating the OGL version of their reference driver, so users were stuck with OGL 1.1 unless the graphics card driver shipped with a custom OpenGL implementation.
So whenever an application uses an OGL version higher than v1.1, it is provided by the graphics card driver and that has nothing to do with DirectX. There is no translation layer in that case (unless of course, that's what the driver does internally, but that's up to the manufacturer).
TL;DR Custom OGL drivers shipped with every graphics card that supported OGL in Windows since 1998.
