Mantle isn't an Operating System, it's a low-level API, similar to what is found on the consoles, it will work on Windows or Linux, it's
Basically up to the community/developers to simply port it over.
Thus you still have drivers to interface with that API, nVidia and AMD have very similar feature sets in order to interface to it. (This is actually how it's done on consoles to, they also have drivers to interface with the Low-level and High-level API's)
That's despite how nVidia and AMD take different approaches to implement such features like Tessellation.

And because the API is open, any hardware company can support it, including a mobile phone running Android.
AMD and nVidia spend untold thousands of man-hours and billions of dollars developing their drivers for maximum performance, they actually have more lines of code than the Windows NT Kernel.

Thus from a developer perspective, regardless if you have an AMD card, nVidia card or even an Intel Decelerator, they're all the same.
They can simply assume all graphics processors will support the same functionality.

Of course, developing to the metal will still provide you with more performance, but even in console-land such things are a rarity, most games use 3rd party game engines like Gamebryo, Unreal, CryEngine, variations of Quake engine, Source Engine etc that interfaces with the low level or High-Level API, it's really only the exclusive big production games such as Halo 4 that are done to the metal.

Basically in the end, it will bring PC's to console-like levels of efficiency for the first time since the 3dfx days when Glide reigned supreme.

Well. Steam has most of the functionality that the console OS's do right now and even one's they don't have like Forums, Workshop (User created content), sharing of digital games etc'.
Leaderboards and Match Making are fine as they are now IMHO.

Also, who can complain about cheaper games? Automagical updates for your entire game library? Larger selection of games? Games with Better graphics and higher framerates? And of course a massive community of 65 Million+ active users, that is growing massively every single year.
I wouldn't be surprised if by the end of this console generation that Steam had more active gamers than either console platform.

With that, there is a Steam sale. It's time to leave mothballs in my bank accounts.

Ibuypower is, despite the crappy name, one of the larger custom-builder PC companies on the web.  My last two gaming PCs came from them, both of which I've been quite happy with.  I would imagine that they'll keep updating the specifications over time.  The whole Steam box thing is, honestly, something of a gimmick.  A gaming PC is a gaming PC.

I don't see myself buying a "Steam box" but I am interested in dual-booting the SteamOS--eventually, that is.  I want to make sure this thing is up and performing well, first.  Valve has never done "support" very well at all and the Steam client has always kind of sucked, so I don't have a huge amount of blind faith built up.  I want to see results before I jump on board.

This also makes the idea of moving away from Windows more of a reality.  It used to be an impossible dream for gamers but now it seems within reach.

Yeah, the SteamOS/Steam Machine thing as specific hardware are really not just major factors here (the hardware aspect even more so given the hardware isn't particularly more fixed an optimization target than PCs in general), compact gaming PCs are compact gaming PCs. Replacing Windows as an OS is really a minor internal political concern for PC gamers, not something that changes things re: consoles. Of course, a minorly more efficient OS vs Windows, and saving the licence fee for Windows if you can manage to avoid it completely (not just dual-boot/v-machine) are great and all, but otherwise nothing much changes: GL/Mantle isn't much different on Windows, and Steam's own service also applies there (and OSX).

Having their own SteamOS ecosystem can only help Valve's Steam as a service though, even if most users are just 'converting' from using Steam on Windows it just accelerates their growth/locks in gamers (relatively) and puts them in a great position as EVERYBODY eventually moves to all-download infrastructure they are established as THE nexus of 'PC gaming' while to some extent reinventing that market. Really, instead of fighting Google over new business sectors and trying to prevent Sony's Blu-Ray format from winning, that is what MS should have been doing to hold onto the PC gaming market, synergizing with XBOX which always was supposed to synergize with Windows gaming, yet in reality there really is no synergy they accomplished. EDIT: Other than imposing the weird Windows8/WindowsPhone tile interface on everybody who has been avoiding those OS' so that they will 'get used to it' and theoretically be more inclined to buy another Windows OS device.

I don't really see it changing things in the console/PC dynamic this generation, but if it works out within their current context they will establish themselves as the nexus of PC gaming completely independent of MS and may well be in a good position to challenge the closed console market later... For next gen (PS5) I kind of expect Sony to return to more heavily subsidized (initially) consoles to offer more bang for the buck and build up their player base, and doing so should be fine if this gen works out well for them and the rest of Sony doesn't go to shit (whether MS keeps trying to compete is unclear, by all signs they will do significantly worse world-wide than lastgen, albeit they can hopefully avoid warranty return problems like RROD). But in such a world, the console makers probably end up looking more like Nintendo now with Wii U, justifiable even more by exclusives. That said, I think Sony is probably in a good position to make such a reality work, with good 1st party devs, good 2nd party relationships, insular Japanese market, AND all their film and TV IP for exclusive game licencing/tie-ins.  On the other hand, Steam may be best placed in China.

This thread was helpful and informative, thanks!

You're probably right that Valve will be taking a small cut, since they make money on the games.  But, we're going to have to wait and see how much retailers are going to want upfront from sales before any company can say how much these machines will cost.

Oddly enough, I'd be more likely to buy a Steam Box than build another gaming PC with a couple of major "ifs."

Unless I start gaming heavily on PC again, I'm just not going to bother putting up the money for a $1000+ box, which is about what I'd spend on a gaming rig (Tom's mid-range "Enthusiast" PC) and I've lost all interest in building budget PCs as an exercise in cost/performance. I have several budget boxes; they're called consoles, and they have fewer problems than PCs and don't require the use of a M/KB or Windows before I can start playing.

I like that it runs on a custom version of Linux, which means no Windows shell to maneuver through and everything tied to using a Windows PC, and also saving $100 on a license. It also means having a special purpose OS specifically for games/entertainment, which is all I need for a gaming PC.

The problem is that not everything that runs on Windows is going to run on Valve Linux. Possibly only a small percentage will.

So the question is, which new release major studio games are going to see a simultaneous release on Linux and which games of my Steam catalog of 130 games are going to be ported? That right there defines the value of the Steam Box to me as a potential consumer. I already have a large amount invested in Steam; no question there. But if it works in practice more like a proprietary box with game ports... I would be better off spending a lot more on building a new gaming PC myself.

indeed, that seems to be similar to the general tone of discussion over at arstechnica's article (comments) on the topic. all about the games. i don't see how this steam machine thing can't help but increase the amount of games available to run natively in linux/steamOS, but the question is how much and how fast? some minor hackery to dual-boot windows on them until windows isn't needed for PC gaming is probably do-able, but that also defeats the point of the simple straight forward 'console' that it's publicized as being and which is a major attraction of it in the first place. 

nobody doubts that awesome nextgen AAA and indy games are coming out for PS4. 

And as a present moment purchasing decision, buying a PS4 now will certainly continue to produce top notch game experiences in 2-3 years with games fully optimized for it.  Buying this lower end Steam Machine now for the price of the more expensive XBone (but without the Kinect) will not really produce that experience because the games won't be optimized for that specific machine's hardware, not to mention it's shortcomings vs. PS4 hardware like the GPGPU on steroids and integrated GDDR memory pool (and the CPU is still unknown).  In 2-3 years perhaps a $500 Steam Machine may offer a better experience thru it's hardware (if the games are there) than the PS4 (at whatever price point it is then, probably around $250) but that is not the purchasing decision you can make now.

It does seem like you should be able to dual-boot regular gaming PCs with SteamLinux in order to mix Windows and SteamMachine games though.

EA says Mantle should become available for nVidia as well, eventually. Also keep in mind that PC doesn't need to be as efficient as consoles to destroy them. We already have graphics cards in the market that outclass the next gen consoles, even on DirectX. If they can become 20% more efficient on Mantle, thats pretty big. Also, unified memory makes things a bit easier for programmers, but in terms of performance, it means that CPU and GPU have to share the same bandwidth. A split pool solution of 8GB DDR3 for CPU + 4GB GDDR5 for the GPU is far superior. Especially when you conside that consoles only use a part of those 8GB of unified ram for games. With that said, I totally agree that PS4 graphics will be absolutely impressive for the next couple of years.

This. Linux has already come to a point where it can be easier than Windows, surely it is for users that aren't system administrators, but also installing it is easy, the only mildly complex thing is deciding a better partitioning than the default one and, in my config, having to manually copy a firmware file for the scanner, due to a licensing conflict preventing some Linux distros from doing it automatically, but even in this case, it's just one file copy, not much different from having to install the drivers from a CD or from the net on Windows. And having done this, Linux standard scanning tools are as easy as Windows ones, but more powerful, if one desires finer options. Printer installation, OTOH, was quicker and easier on Linux than on Windows.

Now SteamBoxes can fill the segment where Linux is still weak, gaming, and streamline what's still complicated for the most casual users, while providing a common platform that like consoles has HW and SW upgrades depending on the interests of gamers and HW and games producers (including Valve itself, obviously), without the interference of an OS producer pushing for major OS upgrades just to sell a new version of the OS itself even when nobody else needs or want it. Obviously the interest of Valve will be strong in this matter, but it won't contain that added factor that in Windows can be totally independent, and even against, the interests of users and HW and SW 3rd parties, Valve won't push a major SteamOS upgrade to sell the upgrade itself, and without this interest, that in Windows can be economically overwhelming, Valve will have a far bigger interest in seeking an agreement on major upgrades with users and business partners.

Personally I'm waiting for minimum, average and recommended SteamBox specs, and I'll keep them in mind, together with XBOne and PS4 ones, when I'll build my next low-cost PC, that will surely be at least Linux-Windows dual-boot, but most probably this time it will also have SteamOS as third option, with the strong hope that Windows, from main PC gaming platform, gradually become a legacy one.