John Brownlee's feature on CultofMac about why he doesn't believe that Apple will ever sell Macs that run solely on ARM is getting passed around a lot this week, and it does an OK job explaining the differences between (and inherent architectural advantages of) ARM and x86 processors.
While he simplifies the differences between RISC and CISC processor design (and I'm no expert), his point basically boils down to the assertion that ARM chips just aren't powerful enough to run full desktop operating systems without losing their power efficiency advantage:
ARM processors are still relatively slow, and unsuitable for the vast array of tasks we take for granted in a desktop or laptop. Compared to the Core i7 in your MacBook Air, the core of Apple’s A5 CPU is similar to that of a 1995-era Pentium Pro. A full-featured port of OS X simply can’t run on an ARM series chip right now, which is what gave us all iOS — a massively stripped down version of OS X — to begin with.
It's a debatable point, as the comments to his article will attest, but one thing stuck out to me: the omission of any reference to Microsoft and Windows on Arm (WOA)--which we know will have a fully-operational Desktop and Desktop programs (Office 15 and Explorer, at least). Related and also worth reading: Ed Bott's explanation of why Microsoft is including the Desktop on WOA in the first place. Bott's rationale makes sense--the Desktop is needed to to perform traditional (and more complex) computing operations like running multiple windows and drag-and-drop file management. That's exactly the type of functionality that Brownlee is claiming that current-generation ARM chips can't run. Except that WOA is scheduled for release this year, and will run on those very chips.