In-Home Streaming could be the feature most likely to kick-off SteamOS adoption. This functionality brings existing PCs to televisions without requiring the user to actually bring the box to their living room. Likewise, to justify purchasing a SteamOS behemoth, it seems likely to me that Valve will allow streaming back to Steam client from Steam Machines.
Video Credit: Devin Watson (Youtube)
Obviously the catalog of Windows games is the most obvious usage for In-Home Streaming but, in some years, maintaining just one high-end computer might dominate.
We will soon find out more about how it works. Valve has just allowed the first wave of development partners (and apparently many others) to the In-Home Streaming closed beta. Youtube videos are already beginning to leak out, or not-leak out depending on the NDA if one exists, which show it in action. The video, embedded above, is of a Lenovo T410 with an Intel Core i5 and integrated graphics streaming DayZ over Wireless-G. It looks pretty good at, they claim, without any noticeable lag.
The floodgates are open. Now, we wait with our umbrellas.