What do you think of Windows 8 as a Human Computer Interaction experience especially for non tablet users ?

When Win8 was first introduced, users really got annoyed with it and found as you said steepy learning curve. There is no doubt in it that flat designs compromises many UX details and expectations. It does't satisfy all the things which are some how the building blocks of UX.

Win8 design is called as MetroStyle, while if we see it, we found it is largely what we called as flat design. 

Before the introduction of flat design, design industry was purely believing in Skeumorphic design, where Apple took the lead and maked a standard where no one could beat it. Apple recently released its IOS7 and it is also based on flat design. When asked to Apple's Designer Jonny Ive by USATODAY in its recent interview why Apple choosed this gradual shift from Skeumorphic design to flat design?

Jonny mentioned that UX is not something which remains constant ever, it is rather a dynamic in nature which tend to change with new technologies, new requirements and new expectations set by users. Apple greately believed in skeumorphic design previously, because users were not much familier with the touch experience, they were rather much well versed with solid buttons of mobile phones. When Apple introduced touch based screens, giving the good UX to users was challenging task where Skeumorphic design helped us greately. Now people are highly eperienced with touch based systems, it is time to give them new experience which will remain with them for a long and will give them great experience and performance.

So when UX always tend to change, it is now matter of a time when users will be much compfortable with flat design and start loving it. User will get softwares/Apps with much better performance and experience., and lots of their job shall be done on their smart gadgets.

Time will come when your smart gadget screen will be having extra smart screens which will tend to change their surface according to things displayed into it. Research is still on and we shall keep our fingers crossed.