Ranked in these QuestionsQuestion Ranking
Pro Biggest game library, and best sales
Steam has a massive game library with over 2,000 games that trumps even the biggest console library.
On top of that, Steam is also famous for having great sales for older, and indie games.
Pro Forever backwards compatible and upgradable
You still get all the benefits of a PC, and all the games you acquire will come with you from generation to generation, with the advantage of having a cheap, easy to set up machine, with none of the fuss of building your own computer.
Pro Open source
The operating system is open source, and the hardware is free to run anything. You can use the hardware you bought as you see fit.
Pro Revolutionary Controller
Steam Machines will come with a new revolutionary controller made by Valve, that is equipped with two touchpads instead of analog sticks.
The touchpads are incredibly precise, and allow for better control for small movement and large.
While it may not be able to dethrone the mouse and keyboard for precision, it is more comfortable to use in a living room setting, and if you find yourself wanting a mouse and keyboard, all games are still compatible with it.
Pro Cheaper than a PC
Because Steam Machines will run Steam OS, there will be less overhead required by the operating system allowing cheaper hardware to run games more efficiently, as well as the cost savings of the operating system being free.
Also, because the boxes will be streamlined and mass produced, those savings will be passed on.
Pro You can still run Windows
Steam OS doesn't have compatibility with all games, but the hardware is still a PC, and there's nothing to stop you from dual booting whatever operating systems you want to play any game you want, and if you already have a windows box you like, you can use the Steam Machine to stream from that computer to the living room.