The list of games that run on Linux (required for a true Steambox) is tiny compared to Mac OS X never mind Windows

True you can install Windows (but that's not longer a true SteamBox) it's a bit like using Bootcamp and saying you are playing Mac games. Streaming from a Mac or PC to a Steambox is interesting but is likely only good for turn based and slower paced games, anything else the lag makes it less fun to play.
Feral have supported Mac specific things for a decade now, over the years things I have personally helped add to Feral games includes:
1. iTunes pausing when dropping into game (un-pauses when you quit)
2. iChat status updates when you start playing.
3. Auto updates built into games
4. Automatic pad support PS1,2,3,4 360, GameCube, Logitech you name the pad and it will likely be supported and automatically be correctly mapped by default. We can even add in support using a plugin system if you pad isn't on the list. I think the list of pads that will map up is around ~125.

5. Retina support in most titles (all new games and every time we update an older game). The ability to scale the UI or not depends on the game engine some scale better than others.
6. Automatic crash reporting to Feral along with Support information generation to help end users if something does go wrong. It's not useful unless you have a bug but when you do it allows you to send us almost everything we need to look into the issue.
7. Screenshots with built in metadata recording the game name, your system spec and hardware and OS/Game versions.
8. Support for overlay notifications from OS X
9. Support for multiple monitors, you can always choose what monitor you want to use.
10. Some games we add in Mac extras here are a few of the examples:
"The Movies" were integrated with iTunes and iMovie so you can do things like use your iTunes music in your games editing.
"XIII" We added in every multiplayer mode and Map into XIII from all the consoles and the PC.
"Batman Arkham Asylum" The PS3 Joker mode was included on the Mac version of BMAC unlike the other consoles and PC.
"Sega Racing" Adding Online Multiplayer back into the game as it was split screen only on the PC.
11. iCloud game syncing for games with small save games (we can't on large games due to space concerns).
I am sure I can think of many more but those are a few off the top of my head, we also fix cross platform bugs when possible.

Making the Mac version better and adding in Mac specific feel and features is something I care about (as you can tell from the list I made

). I like to think we make Mac versions of games we don't just make Mac ports. It's a subtle difference but one that I think goes to the heart of why we do what we do.
The list of new Mac only extras that we want to do is long and we add them into our standard list of features one by one. If you have a nice feature that would make our games better just drop me a PM, I cannot guarantee it will be implemented but we do read and listen to our user base on this and other forums when looking for ideas on what we could improve or add to our games.
Edwin