Many Windows games need OpenGL, like UT2003, Quake 2 (and all games that has the Quake 2 engine, like Half-Life and Tribes 2), Quake 3 (and all games that has the Quake 3 engine, like Medal of Honor), etc.
D3D had more effects, details and anti-aliasing rate then OpenGL a few years ago, but OpenGL has catched up, and provides the same detail that D3D delivers, except that D3D has a higher anti-alias rate.