BTW, by posting that link I don't mean to suggest that Win8 is perfect or you can't complain about something being non-obvious in an interface, just that the "haha, it's not intuitive! therefore it's badly designed, this proves it! end of discussion!" "argument" is lame.
A UI's key purpose is to expose internal functionality to the user. If the user knows that there is a feature in the product and cannot find out how to invoke it, the UI has failed.
It is all very well to say that the UI has other jobs as well - it's important to be consistent, to look good, to localize well and to avoid unexpected transitions. But fundamentally the first job of the UI is to expose functionality.
If the UI makes critical functionality less discoverable and requires users (like Dr Herbie) to have to read a manual or a search engine to use your UI, then as a UI designer you have failed.
This isn't a subjective measure. Dr Herbie wanted to do something, and Windows8's UI got in the way of him doing it compared with Windows7, because I presume that Dr Herbie had no real difficulty discovering how to turn off his Windows7 machine several years ago when he first got one of those.
That is an entirely objective measure by which Windows8's UI is worse than Windows7's.