"I actually think that Direct3D is a rather better API today," he told bit-tech. "Microsoft had the courage to continue making significant incompatible changes to improve the API, while OpenGL has been held back by compatibility concerns. Direct3D handles multi-threading better, and newer versions manage state better."
That being said, Carmack won't be switching away from OpenGL anytime soon. Despite the advantages of DirectX, OpenGL is rooted deeply in the company's game and tool code and transitioning to the Microsoft camp would involve a lot of work. It could complicate supporting platforms like the Sony PlayStation 3 and the Apple Mac.
"It is really just inertia that keeps us on OpenGL at this point," Carmack said. "OpenGL still works fine and we wouldn't get any huge benefits by making the switch, so I can't work up much enthusiasm for cleaning it out of our codebase. If it was just a matter of the game code, we could quite quickly produce a DirectX PC executable, but all of our tool code has to share resources with the game renderer, and I wouldn't care to go over all of that for a dubious win."