Representatives from the team at Valve take a look at the difficulties they went through in porting the Source game engine to Linux. They discuss various tools they used to do the port, and how switching from DirectX to OpenGL was difficult but may be a hidden gem for game developers.
They state, unequivocally, that OpenGL has most if not all of the capabilities of the current versions of DirectX, and yet newer versions of OpenGL are fully functional on older computers- like the 38.73% of computers still running Windows XP. Someone running a newer GPU on an older OS would still be able to experience the benefits of a modern gaming engine, but wouldn't have to go through the trials and travails of upgrading their operating system and reinstalling all of their software in order to merely upgrade their game visuals. Using OpenGL instead of DirectX will also make it easier to port to other platforms (like OSX and Linux), and further down the road, porting to mobile.