Really, the OpenGL API is almost multiple APIs these days if you're using one flow of how to render your primitives, the other flow is downright toxic to your rendering engine (in that it's going to muck about with your GL state in ways that you shouldn't burn mental energy predicting). There's also a bit of a cultural issue if you learn OpenGL from the 'red book,' you'll learn the function-per-primitive API first, which is precisely the wrong tool for the job in making a performant game. This is demonstrated by some key features necessary for performant games (clock synchronization to avoid 'tearing' comes immediately to mind) that are core to the D3D spec and extensions to the OpenGL spec. OpenGL's origin was as a vector-based drawing tool for CAD applications which was repurposed to games D3D was targeted for performant rendering with games as a specific target.
Quite a few of these issues (especially in the "Too many ways to do the same thing" category) relate to OpenGL's age as an API, which is something Direct3D's design was able to learn from and improve upon.
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