Heh. The very point of Windows 7 is how quicly it comes out. Both 2k and XP were around for 4 years before a succesor came, while Vista haven't been for two, and Windows 7 is just around a corner... it says something about vista IMO. With all my conservativeness about OS and beeing extremally unwilling to downgrade to a new version (i always felt that way), I was unable to just wait XP over, and i'm pretty confident i'll go from XP straight to 7. Also, AFAIK now major corporation upgraded to VIsta, and thus it has been a commertial failure. I really don't know what you mean by saying that vista puts memory to good use, for me best it could do would be leaving as much as possible to other programs.
As for D3D and OpenGL. I don't know about now, but back in the early 2000's you could really see the difference, while running the same apllication under both. OpenGL looked way better, so there are good reasons to use it. Also, while many games use D3D, it's far frome every one, biggest example being ID games wich from the dawn of #d Accelerators used OGL exclusively