Valve's Steam Machine platform hasn't really taken off, and it's not just because of the hardware. When few developers are willing to release Linux-native versions of games to run on Steam OS, there just isn't a lot to play. Thankfully, the company may have a way to bring more titles to the platform... sort of. Reddit users have pointed out that Steam's graphical interface files now include a section with not-so-secret references to a Steam Play feature that would auto-load "compatibility tools" to play games "built for other operating systems." In other words, Steam OS might include some kind of emulation.
This wouldn't guarantee playing anything and everything your Steam Machine could handle. Only "supported titles" would work right out of the gate. You'd have to toggle a setting to enable Steam Play for other games, and you'd receive a warning that the tool might prompt "crashes" and break save files.
There's no guarantee that Valve will roll out this feature in the first place, let alone the near future. If this is emulation, you also wouldn't get stellar performance barring some surprise technology that minimizes the usual processing overhead that comes from running Windows apps in Linux. This does reinforce Valve's claim that it's "still working hard" on Steam OS, however, and it suggests that you might not have to give up quite so many games just to play on a Linux-powered box.