If you consider Mac OS X then it has never been the preferred platform by gamers. Although, it does go on to support OpenGL 3D graphics framework! The concern here is that graphics hardware drivers for Mac OS X are not optimized as much as you would find them on Windows. In the Open GL benchmark comparison that took place between Mac OS X10.6.3 and Windows 7*64 by Phoronix running on the same Apple hardware that was equipped with 9400M with Mac mini Windows 7 trounced Mac OS X with raw speed.
Valve is of the opinion that when you look at it according to performance then there is a huge scope for improvement. Valve developer Jason Mitchell states that they will certainly be able to catch up. Apple is motivated to work with the company and same is the case with them. They are going to work out a deal with hardware vendors. This is the same hardware but it can boot up to different OS and have different performance characteristics.
The founder of Valve Gabe Newell states that Mac performance and its support GL shows a great opportunity for the company. They are not as mature whilst working on the Mac as when they work on the PC.
They have been sending games for Windows for years whereas they have only been sending games for Mac only since a week now.
Keeping aside the performance, Mac has several other benefits too. it has only got a model and OS version, programmers are able to create bugs and profile hardware performance on Mac. Whereas Windows derives its benefits from a faster driver updates. Mac users on the other hand will have to wait for Apple to roll out a point release. But, the proliferation hardware and software configuration makes it a difficult proposition to narrow down on the exact problems.
Newell also mentioned that the surprising thing was how very stable their games were on the Mac. The early data that was available from the Steam client showcased that the Mac is five times more stable when compared to Windows. This is when the use of metrics of minutes pass played compared to the number of crashes.
Newell also mentioned that the podcast had graphics which were not a concern as was the finding out of ways to enhance the overall customer experience. They are moving away from graphics approach to more of a service platform.