I'm hoping that, while they allow anyone to use SteamOS for their system, they create a "branded" system by which to easily measure system specs for quick eyeballing of whether a particular Steam Machine can run a particular game. The way I see it, they have yearly specifications and a ranking system per year. It can be simple numbers 1-5, or tier like Start, Mid, Pro. Then it's a rolling designation, with the idea being that this year's Pro should be roughly equivalent to next year's Mid, then the following year's Start. It also means that games can target only that year's specs on their box; instead of having to say something like "Supports Windows 8, 7, Vista, XP", a game released in 2016 can list SteamOS support as "Supports 2016 Pro and beyond", so gamers that pick it up later will know it also works for 2017 Mid, 2018 Simple, and then everything after that. Maybe have it simplify it and have just a High and Low tier, or go even further and have a minimum hardware requirements to be "Certified SteamOS-Compatible 2015."
This is a lot like the Windows Experience Index, but easier to understand IMHO (their system is an addition one, in that they'll increase the max score over time as newer hardware becomes available; right now it's still 7.9 for Win8, which was the same max for Win7).
With this kind of system in place, they can have the best of both worlds: Those who want more esoteric hardware options or roll their own can do so (SteamOS would include a utility to give your system a rating, though it wouldn't be "certified"), and the major hardware manufacturers can target one or a few of a set of hardware specs in order to improve the price point through mass production. Depending on how high the "minimum" specs are, they could even make a cheap-o or super-small model by targeting a prior year that can still run the biggest AAA games.