There are some idiosyncracies where it is clear Microsoft wanted so badly to introduce a new paradigm, they stupidly sacrificed usability.
One example is the lack of a startup button. I didn't say start menu, but I see no reason why they couldn't add something, anything, to indicate where to go for the startup screen. Or the charms menu, which have a number of ways to be triggered, some of the inconsistent. It's kind of a chore to do things as simple as shutting down.
For most of us (I imagine MR users are pretty savvy), it's not too hard to figure out what's changed. But imagine the average user (my parents), who already struggle with Windows 7. How the hell are any of them supposed to know how to access these menus?
This will be a future phone conversation for sure...
Me: So go to the bottom left corner for the start screen, where the start button used to be.
Mom: There's no start button anymore.
Me: I know, but just go there and click it, trust me, it'll work.
Mom: But there's nothing there, where do I go?!
Me: To the bottom left corner. Just do it. You can't see it anymore, it's invisible, but it's there, I promise.
Mom: Okay. A screen came up with my apps. How do I get back to my desktop?
I agree wholeheartedly that this is an excellent mobile/touch UI, but pretty useless (or worse, obstructive) on traditional PC. I wish Microsoft implemented this as a sort of software layer that could be enabled or disabled, so they could still have the best of both worlds in a single OS, but many of the annoyances could be avoided on regular computers.
For some reason people on Windows find it hard to imaging clicking the VERY corner. I tell people click in the very corner, and it always results in "nothing is happening". You then have to say very clearly, ALL THE WAY IN THE CORNER.
It makes sense that you wold use the corners, but its strange for people for some reason. After people get adjusted and learn how switching/start menu works, its usually fine though.