Speaking as someone with actual experience in this - DirectX is a monumental improvement over OpenGL; there's more to it than just being able to look at the underlying code (and frankly, 90% of both are just shims to pump commands into the drivers which is where the real magic happens -- drivers which generally tend to be very closed.)
Start with;
- A (arguably) better shading language (HLSL vs GLSL - I tend to find HLSL is easier to work with)
- A scenegraph representation that is not built around a 1980's pre-acceleration stack based renderer. (e.g. Objects are represented as full OO objects - complete with materials, rather than a matrix pushed onto a stack followed by a few vertex buffer array calls)
- A suite of excellent debugging utilities - which give you useful feedback (looking at the code != the value of a good debugger suite)
Leaving the 'free software' arguments aside, DirectX is better - by virtue of the fact that it's kept up to date with modern programming techniques. OpenGL needs to be replaced or at least significantly overhauled -- something which Khronos has not in the past actually been willing to do. OpenGLES improves the spec marginally, but it really needs a replacement to be on par with the ease and power DirectX offers you with far less effort.
That said; if anyones doing things at the DX/OGL level, they probably need to be hit over the head - anyone serious is using one of the many decent engines out there.