From article Carmack: Direct3D is now better than OpenGL.
John Carmack, co-founder of id Software: "I actually think that Direct3D is a rather better API today. ... 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."
Also from article: "While newer versions of OpenGL have kept up-to-date with some of the features found in DirectX, they usually have to be implemented via extensions, rather than the main API.".