The VentureBeat interview [1] makes it look like gaben is referring to the changes in app ecosystem (eg. integrated Windows Store with 30% margins) rather than a specific technical problem. Although I suspect a major games studio coming to Linux might also enjoy the ability to poke around the source for the few graphics drivers which are open.
1. http://venturebeat.com/2012/07/25/valves-gabe-newell-talks/