I think there's still a piece of the puzzle missing here. As Cobra pointed out, they're not really standardizing anything, just coming up with a bit of a more specified Windows Experience Index sort of thing and maybe offering three hardware tiers around it. But at the end of the day, you're still looking at a gaming PC hooked up to your TV, not a streamlined and standardized pc/console type thing....which is where the appeal is.
From where I stand, they'd be much better off hammering out a standardized design based on that middle tier, reap the benefits of the increased scale of production on those parts in order to offer a lower price point, and work on getting publishers and developers together to set that hardware as a baseline for future PC releases. People who want higher end can just play on their PC, as that's not going away, or they can just hook their PC up to a TV, OR they could buy the $99 peripheral.
That really seems to me like the only point it doing this. As it is, it sounds like they're just trying to develop a front end....which they already have with Big Picture Mode. I really, really don't get it - It's almost like they're just utilizing the bad parts of PC gaming and basing a system on that. Not to mention, I don't know what the fuck $300 is going to get you without economies of scale in production and a system setup where the provider (Valve) is willing to take a loss in hopes of future profit through licensing fees and sales, and it doesn't seem like they want to set it up that way. Seriously, you can't build that decent of a gaming PC now for $300 - and that's just looking at cpu, gpu, ram, and the mobo. There seriously has to be some sort of plan to offset the costs, apart from just reaping the rewards from selling games through Steam.
And finally, by going through Linux, they're cutting off 90% of the pre-existing PC game catalog. Sure, I'm sure if they focus on Linux and provide developer incentive that number will get better, but lets not kid ourselfs, a lot of past, current, and future games still aren't going over that way with the plan they have now.
It seems like they're over-complicating what should be a simple plan:
-Release standardized PC hardware to be hooked up to TV at a reasonable enough pricepoint to attract people who find the high cost of pc gaming a barrier
-Provide front end and store
-Use the baseline metric they've made here to entice developers and publishers to port more games to Steam/Linux. This serves to help Valve get away from windows slowly, as well as increase catalog and marketplace acceptance.
-Ensure users that their new Steambox systems will be able to play Steambox games for years without having to upgrade.
Simple plan that could very well make Steambox the template for PC development. Options still available for higher end PCs, but the safe bet for developers would be to make sure that their games run on Steambox standardized hardware flawlessly. No idea why they aren't doing that, it's almost like they're only try to sell this to people who already game on PCs...which is dumb.