Here's what will make this attractive to consumers: Right now, if you want to play a game both on your PC and in your living room, you effectively have to buy it twice. Or you have to make a decision for one. Steam's proposition is that you buy it once, and you can play it on any of your computers, even the one in the living room that has a gamepad attached. Heck, even if you're an exclusive console gamer, it will be hard to pass up the psychology of getting more for your money.
This isn't just another console, it's a new value proposition entirely.