Just a few points currently:
"Tap this one, and presto, you are back in the familiar Windows user interface, though this is as frustrating as ever if you try to run apps using touch alone."
Why on earth would you? Since you would surely launch the traditional desktop if you *intended* to interact with the device with mouse and keyboard?
"A critical factor is whether Metro apps will cover enough features so that tablet users will rarely need to venture into the desktop."
They will surely venture into the desktop when they wish to use the aforementioned keyboard and mouse for those tasks which are not suited to touch. For example, a usage scenario: I have my shiny new windows fondleslab sitting in a dock on a shelf in our TV bench connected to our TV via our receiver and I wish to post some more wisdom at El Reg from the comfort of my arm chair. Am I going to use the touch UI? Of course not, I will launch desktop and use my wireless keyboard and mouse just as I am doing right now as I type this.
I note that several on the thread appear to have physic powers inasmuch as they already "know" that Win8 is going to be totally pants. I have a seriously radical suggestion. Why don't we wait until we have actually had some hands-on time with a Win8 build (at least a stable beta or better still the RC installed on a device with a touchscreen) *before* we form an opinion about how well its been implemented? You know, something so seriously cutting edge as *knowledge-based* criticism.