I tend to disagree with most of the comments on this one.
The ARM SoC will be co-designed by three companies and, being a future chip, will surely take advantage of smaller nm manufacturing processes, more cores, and new strategies for multicore architectures.
I think that the Windows 9 core will be used, not 8. Windows 9 might not even be available for developer preview by then, but we're not talking about the full- fledged OS here, it's just the core. Since the job here is not to operate a PC, but a gaming console, the software around the win core can be a bit simpler (as in "unburdened" or "unbloated"), and that means faster development time.
Performance-wise, nothing can stop them from customizing the chip, making it more powerful. It's ARM allright, but it's only made for the XBOX, not for general, multipurpose usage. They're already planning for specialized ("assistive") cores for graphics, AI, physics etc. Equally important is the fact that the software developer, is also the chip designer, translating to less compromises and tighter integration between h/w and s/w. Now, considering M$'s efforts towards better and more efficient utilization of multiple core chips, especially in Windows 8, this upcoming console looks very promising. Let's hope M$ will make it right.
The most important part for me, is the development platform. I hate ports just as every other PC user out there. Ports, not only take away from PCs the better visuals, but even worse, they take away the very essense of gaming, the gameplay. That's where fun in gaming mainly comes from.
If, by using the Windows 9 core, they can make development transparent between PCs and the new XBOX, then I think that's really good news. Normally, developers will only need to consider the Windows 9 core when programming, not the underlying architecture. That's what M$ has been doing on PCs so far, and I believe that's were they're taking XBOX, too. I'm sure many disagree with M$ on that, especially developers who like to push hardware to the limits (id software is one I guess), who will lose their freedom of "talking" directly to the h/w (maybe even more), but that's a different story.
As long as that's what they're aiming for, I think we should be somewhat (yes, I hate it when developers have to compromise, like they do with DirectX) happy, because this means better ports. Not really ports, but if developers still need to reprogram controllers from gamepads to mice and keyboard, I'd rather call it a port.
One thing is certain, the world of computing is undergoing dramatic and radical changes. I cross my fingers.