One feature that's going to really benefit Steam Machines and some home setups like mine is the ability to stream games on Steam to another machine in the house. Well, the beta is now going out to some folks and I got my invite this morning.
This page details what needs to be done in order to get it all to work. So now, Linux based SteamOS machines can be used to play Windows based games provided the host computer is Windows. This would essentially make the Steam Machine a thin client.
I've got about 4 computers around the house with Steam installed and it would be great to have one centralized machine where I can stream games from. Since it'll be an internal network, performance should be good with wired networks. Wireless might be a hit or miss though.
One interesting note is that it may stream non-Steam games in your Steam library, although it's not officially supported.
This will really simplify my setup and enable older machines play games that I couldn't before. I can't wait to try this out tonight.