I happen to like the "feel" of OpenGL more, which isn't something I can explain well to those that aren't programmers, but I don't think it matters much at this point which API one chooses. More or less, you have feature parity between both APIs, both have made a handful of design mistakes, and both do a few things in slightly more pleasant ways than the other. Of course, the unforgivable thing about Direct3D is that it only works on Windows. If it was truly cross-platform, though, I wouldn't really object to using it. But since you write OpenGL almost everywhere, and Direct3D almost nowhere, it seems like a false choice to me.

Quoted in Luboš Doležel, "Interview: Ryan C. Gordon" AbcLinuxu.cz (2011-03-08)