The DirectX/OpenGL issue always remains the contentious issue. While many people do blame (and rightly so in a way) the hardware manufacturers, people often do not take into consideration that game programmers do not think beyond DirectX because Microsoft is by far the most popular gaming platform on the PC. Porting Direct3d games will simply mean a complete rewrite of the underlying game engine code at the very least.
The driver performance issue has been much commented upon and I would skip on that.
As far as driver development efforts go, ATi has acknowledged Linux so far, but NVidia's policy seems to be much more proactive in supporting Linux. I wish I could dump my ATi card and go for another NVidia card, though I suspect it will not happen anytime soon.