While nobody can know for sure how things will play out next year, I have a feeling that gaming on OS X may benefit as a side effect of SteamOS being successful if it is.
It has to be considerably easier to port a SteamOS game using OpenGL to OS X than it is a Windows game using DirectX. The difficult work of moving from one graphics API to another is already done. One thing I could see as being problematic however is the speed at which Apple will introduce OpenGL changes and also graphics driver changes to support gaming in OS X versus the speed at which such changes would likely tend to be implemented in SteamOS and Linux.
That isn't to imply that porting anything is trivial but it certainly has got to be a lot easier so we might make out pretty good on Macs with more titles coming our way than might have otherwise. This all assumes that SteamOS takes off but considering Valves position with the Steam store if anybody can wrest control from Microsoft it would be them.
I've thought for a quite a while that this was a smart response on old Gabe's part in anticipation of Microsoft having their own app store built into Windows where games would be sold and may have been a potential threat to Steam. SteamOS not only addresses that but also moves Valve into the console space while at it.
Anyway, it is going to be interesting to see how all of this plays out. In the meantime, between all the Mac stuff I have and a planned Playstation purchase, I won't be hurting for excellent entertainment while I observe the show.