The major difference between Direct3D and OpenGL used to be that OpenGL is a direct mode API only, while D3D had a retained mode interface back around version 6 as well as a hideously clunky direct mode API. Nowadays the retained mode API seems to have been phased out -- not that I care.
Several pretty good retained mode libraries are available for OpenGL; the most well-known are probably SGI's OpenInventor and OpenGL Optimizer, but several free alternatives have been released as well (OpenRM, sgl).
It is rumoured that SGI, at one point, had the entire OpenGL 1.1 rendering pipeline implemented in silicon. That'd really peg my neat-o-meter.