It's not ridiculous - it's a simple fact for me. In it's default state, Windows 8 has a UI which is unusable for me. Maybe you have different requirements to me, that's all well and good, but you can't take your experience and imply that someone else's is void because of it.
The start menu is full screen? Hardly just that. If that's what it actually was there wouldn't be an issue.
I honestly don't know where to start with what's wrong / harder to do. It's more of a case of what does it do right for me (which isn't much).
It's disjointed in almost every way. Launching an app? Is it on the desktop or metro? Jumping between them is a disaster, a simple press of the Windows key doesn't take you back to the desktop - it takes you back to the previous app. You end up with this weird situation where launching the default browser will open in metro, yet you fire up Photoshop and you're on the desktop. The usability of using the browser and a desktop app at this point is zero. You have two distinctively different ecosystems that have their own working space.
This is bad with a lot of common applications, but when you start to code....jeeeeeze...I can't begin to explain the horror of trying to use an IDE like Visual Studio, a browser and something simple like a music player with Metro in place. It's horrid. I am constantly fighting the UI and in many cases, trying to figure out how to do things (Don't even get me started on the abysmal 'charms' menu)
This disjointedness is bad enough for home users, but drop it into a work environment and you have an entire host of different issues.
And to top it off - some apps come with advertising built in from the MS Store. Not downloaded apps. Default ones. Hang on MS... I just PAID for this OS, why are there adverts in your default installed apps?
There are many other issues with have annoyed me (such as dropping Aero), but they are not usability issues as such.
For me, there is no "short while to get used to" as I use the OS for more than browsing and mobile like apps. It's unusable for me in it's default state when it comes to multitasking and coding, to claim otherwise is ridiculous
Then again, if you think Metro is just a full screen start menu then you probably don't use your machine in the same way as those who have issues with it.
I just wouldn't use the OS without Start8.
-edit
There are a host of very respectable websites explaining the issues in depth. There is little point me going through them all again, because they're spot on:
http://www.zdnet.com/what-not-to-buy...ps-7000007557/
http://www.itproportal.com/2012/10/2...ktop-disaster/
http://www.osnews.com/story/25951/Wh...with_Windows_8
Contrary to popular belief, Metro is not a replacement for the Start Menu. Metro is a replacement for the Explorer Shell. The Explorer Shell itself has been turned into an application. Traditional applications run within this Explorer Shell, and cannot be managed from Metro. In other words, the Explorer Shell has become an application with a multiple document interface, running in Metro.
This, right here, is the main reason why Windows 8 is such a pain to use with a mouse and keyboard. You can't directly switch to a desktop application; you always have to first switch to the Explorer Shell, and then switch to the desktop application you want running within the Explorer Shell. This is a convoluted way of using my computer, especially since Metro itself isn't mouse-friendly to begin with, with finicky hot corners and UI elements that are too volatile.
Consider this. To switch to a Chrome browser tab, you have to: switch to the Explorer Shell in the Metro application switcher (and hope this doesn't go wrong), switch to Chrome in the traditional taskbar, and then switch to the right tab within Chrome. This is insanity. Whenever I go through this in Windows 8, I hear this playing in my head.
It's not a technical issue. Microsoft could easily integrate the two much more efficiently and more fluently if they wanted to. No, the real issue is that Microsoft doesn't want to, because (and here's the pill that's so tough for some to swallow) the Explorer Shell is being deprecated. It's dead. It needs to be cumbersome and unpleasant because Microsoft hopes this will make users demand Metro versions of their favourite applications.
Trying to shoehorn a tablet/smartphone user interface paradigm into a desktop/laptop computer is just as flawed as trying to do it the other way around. Unless Microsoft has some sort of grand plan to reintroduce proper window management into Metro, I don't see how I could ever get any serious form of work done with it.