Hello ChaosUnlimited,
Great to see you have an interest in the technical features of the FS2Open engine.
The best way to learn is by building and reviewing the source code, and for this engine all of that is publicly available.
A couple of your points have been responded to above, but thought I'd also add:
* 64bit: We already build 64bit executables on OS X and Linux, so there's no fundamental technical issue.
* DirectX: The engine actually used to be DirectX only when released by Volition. As part of our team's work to provide cross platform support, an OpenGL renderer was added. In the interests of maintainability, the DirectX renderer was subsequently dropped as it add too much complexity.