Might as well add a #8 to my list. Vista can't deal with multiple video cards that use different drivers. IE, you can't install an ATI 2600 XT and an NVidia 8800 GT into a system and have monitors attached to both function.

OS X, and XP, not a problem. OS X even lets me move 3d apps between the screens seamlessly.

Now I have a reason to try and get XP 64 working on this Mac Pro for Boot Camp. The lack of Apple supported drivers for XP 64 caused me to try to live with Vista for wintendo purposes, but it seems it fails at that task too.