This is actually quite dangerous (or great for Linux). Just imagine that you cannot install any kind of software on your windows PC without going trough the app store. (No, you mac people are lost already :P)
My understanding of windows 8 is that you can install what you want on any x86 system, but not on their ARM systems. This guy seems to be saying that this is not the case, but this guy's piece is a bit too complicated for me to work through at the moment. Who wants an ARM PC? Not I. Am I sad that I might not be able to get a Metro app outside of the widows store? Nope, I haven't heard about an app that I want yet. I know people who have installed the windows 8 pre-release and installed programs on it, so I think this guy isn't being entirely clear about what it's going to let you do. There are so many proprietary programs used by business that making people install only through the app store would be suicide. If this was true I think Microsoft stock would have already gone down by at least 25%.
I think most of this talk of Windows as a closed garden is pretty speculative. I don't really buy the post PC world talk, but I can see why people are worried that about the decline of general purpose computing. The more PC's become exclusive to high performance users the less things like Metro will appeal to that market segment. I don't know anyone who can edit their registry who has any interest in Metro apps and I think Microsoft is going to have a hard time to get casual users excited, let alone the enthusiast crowd (remember how we were supposed to be excited by those sidebar gadgets in Vista?). The ARM walled garden is just an attempt to create a "user experience" to rival apple, I don't think they can pull it off. Metro on ARM is going to die on the vine and with it hopefully the idea that Microsoft needs to tailor user experience. I think the next five years will be interesting for ARM and x86. Intel is just dipping its toe into the mobile chip market with pretty impressive results, I think they will be very competitive soon.