And from the looks of it, it still looks better on the PC. My bro says its because DirectX is much better compared to the aging OpenGL engine. I wonder how true that is.
I just wanted to clear this statement up for you (not in a mean way, just general information).
OpenGL and Direct X are not equal. OpenGL is a graphics API, where as DirectX is a bunch of different API's rolled into one. (API = Application Programming Interface, its how you program stuff to talk to hardware).
OpenGL can be compared more directly to Direct3D which is the graphics component of DirectX. Also OpenGL has a newer release than DirectX (unless you count DirectX 11.1 but thats only in Windows 8 beta as far as I know).
Now, as far as graphics capabilities, both are about equal, sometimes one comes out with something the other doesn't have and vice versa. For most applications you'll never see a difference.
OpenGL is used by almost all applications out there. High end 3D packages like Maya and Houdini use OpenGL. OpenGL is prominent on Linux, Mac OSX, and Windows.
DirectX is Microsoft specific and is used mostly for games. I am familiar with programming for both OpenGL and DirectX and I personally like OpenGL better (from both a programming standpoint and a performance standpoint) but DirectX has its merits too. Sometimes graphics card companies have specific DirectX drivers that help improve performance but its usually negligible.
As for Diablo 3 they looked and played identical for me on both OS's. You may have a different color profile on your monitor on one OS vs the other which is why you'd be seeing a difference.