GpuTest 0.4.0 for Windows, OSX and Linux
One difference between D3D and OpenGL is that the former is using an object-oriented API. All API calls are virtual rather than being plain C calls like in OpenGL. The main advantage of this is of course flexibility. The runtime can easily provide many different implementations and hand you back any one depending on your device creation call parameters. The obvious example of that would be the debug and retail runtime.