Okay guys, here are my thoughts.
Before any wild speculation about competition with consoles or keyboard vs. controller issues or streaming options begin, we should really wait for the other two announcements. I'd agree that one of them is more than likely going to be the Steam Box as it's been basically confirmed by now, anyway. It only seems logical that they'd want to make some nice new hardware to go with their software. As for the other announcement, who knows? Although if it is HL3, which I very seriously doubt, I'll eat my trousers...
As for SteamOS, from the very little we know about it at this point, it seems like a good thing. An OS that is designed for games and is a FREE alternative to Windows sounds good to me! As long as you're not missing out on games because of compatibility problems, it's a solid plan. It would seem strange if they released a Steam OS that couldn't play any game a Windows or Mac machine could, surely? It will never replace Windows, that's for sure. But that's not what they're trying to do.
The way I see it, the popularity and success of the whole Steam Box/ SteamOS endeavour relies upon: a) The backing of developers - if you can't play the vast majority of new releases, who will use it? b) Good TV and demand services - if it's going to be the only box in your living room, you're going to want access to everything, right? c) Price - Simple as. If it's more than a console, then what's the point?
I don't see game returns/trade-ins as a problem. People seem content with Steam the way it is on PC's now, changing things is unnessecary. Besides, the whole "Family and Friends Sharing" thing is their solution to this, which I'm all for, depending on the implementation.
Valve are known for trying crazy things and I'd say that usually, they succeed. They made their goofy, five year old arcade-shooter into a F2P and now it's a unique international hat-economy, earning them who knows how many millions of dollars a year, for goodness' sake! Trust them, guys. They know what they're doing. When they reveal what's going down, I'd be surprised if we come out dissapointed.