The biggest thing to me about people such as this suggesting Valve don't know what they're in for is that this isn't just a sudden decision for them. How long were there rumours that Valve was experimenting with hardware before they began to hint at a Steam Box, and how long was that before they officially announced they were going ahead with it?
I just think, regardless of whether people like the idea or Valve in general, they aren't a company to skimp on informing themselves before making decisions. Everything they've done has been a gradual, calculated move. There's so many concepts they toyed with in TF2 before deciding they had something and spreading them into other games. Things that are big now, like the workshop.
Big Picture mode already tested the waters for how Steam might operate in the living room, and it seems like the interface is well-thought-out. The lotus flower keyboard concept is amazing and I don't know why the other consoles didn't come up with something like it sooner; its just simply straight up faster and easier than the current standard of throwing up a normal keyboard layout and navigating back and forth across it.
That to me shows that they've put serious thought in. If they're really going ahead with a Steam Box, I quite believe they are fully prepared.
As a side note, I love some of the ideas they've talked about alongside the Steambox. Things like using biometrics to measure the player's physical state in order to tell when they're physiologically excited and riled up, then using that to try to pace a game experience. Not to mention the stuff about eye-tracking and VR. That sort of stuff seems a lot more interesting than motion controls - and the best the established console brands can come up with is copying each others' motion controls? If Valve's ideas bear fruit, they'll set themselves up as the innovators who the rest will be trying to copycat just like they did with motion control.
Though I'm not sure if I'd get a Steam Box, I do hope it succeeds, simply because Steam's dominance of the PC market means if they succeed with a sort of standardised PC (with tiers as has been talked about; the theoretical low-range, mid-range, and upper-range variations of the Steam Box), they could unify PC developers into adhering to some such standards - finally removing the biggest barrier between PC and console gaming: compatibility and stability issues.
Then again it may not work out anything of the sort. But if nothing else, it's very much a step in the right direction, and Valve seems to be the only ones willing to take it.