I know I'm going to get down voted a lot but I want to express my opinion regarding this topic since it looks like such an important topic to many.
Background: Some cumbersome UIs
I'm an avid multi-OS user, I use Windows and Linux with the massive amount of desktop environments available for the later.
Before going to the “Windows 8 Story” I want to tell a little story about something funny that happened to me in Linux so everyone can understand the context for the text following this.
Not so long ago (around 8-10 months?) I started to use pure Openbox (nothing more, just the WM) as my UI for Linux. My “native” DE is KDE and I'm used to hot corners since KDE implement them.
As many may know (and as many may not know) by today's standard a pure Openbox environment can be considered as “unusable”(it's only the window manger and a little menu to be honest, there's no background picture or something else), but I started to use it because I liked the lower resource usage (the whole OS just needs 200MiB of RAM).
Eventually I became so used to the interface (and the really, really, lower complexity involved) that when I went back to KDE it felt really unintuitive.
As long as you get used to something everything else will feel uncomfortable.
The only real argument that you can make about Windows 8 is the context switch one as it happens that we humans are really, really bad at it.
However you can say the same about multi-tasking to certain extend (we were single-threaded creatures last time I checked), but there's no escape to it.
Maybe it's because I can adapt to new things fast or I like change?, I'm young, older people may face troubles to retain new information or usage patterns.
I didn't have problems with GNOME 3 either – if you think Windows 8 was a huge change, ask Gnome 2 users how they felt, they didn't have the old desktop simultaneously, but that's a completely different story, however the reasons were the same –
Windows 8 Story: Not bad for me at least
The first time I used Windows 8 I knew a lot things before hand.
I'm using the OS since the Dev. Preview and back then you still had the start button and a “mini start-menu”, so I learned the basic OS UI and when I finally had to face the real UI (Consumer Preview) adapting to it wasn't a big problem.
I can't give you the opinion of someone that faces the whole (final build) OS for the first time, trying to get into the shoes of someone else isn't my goal here.
Before someone says that I don't do real work on my computer. I use a DAW for composing to picture so I open the DAW (2-3 windows), plug-ins(10-20 windows), a third-party file manager (orthodox so 1 window), a notation/score editor(1 window) and obviously the video window.
To be honest I don't have a lot of things to complain about the OS since (for now at least) you can still use the desktop UI to work.
I can't find a simple thing that stops me from doing the activities that I did in Windows 7 or difficults the same activities for example, however it's my case and my workflow; out there sure is someone that got the bad side of things in his face.
Modern Apps: Just too good in theory to be real right?
Respecting the Modern Apps I have a mixed impressions.
To begin with I found apps that can help me and adapt to my workflow however those were incomplete or had the Modern Apps' main problems:
1. They crash under “heavy pressure”; for example, no matter which twitter client I try if I go out of the app for 1 min and then go back my timeline will simply overload the app and it will crash.
2. The apps suspend their execution once you get out so there's no way to really get then to be running in the background reliably; no matter if you select it in the app settings (even Microsoft's own Skype fails) so for real (or heavy) usage they are useless.
Microsoft: Some huge compromises
Now, I understand people hating Windows 8 since depending in their workflow and workload they may get a huge hit in productivity or may not have the time necessary to adapt to a new OS with a new UI paradigm.
I believe that Microsoft knows many of the problems that Windows 8/8.1 will encounter or at least there has to be someone inside Redmond that thought about those problems.
But it's kinda complex, I mean lets see the scenarios:
A) Develop a separated version of the OS for tablets or escalate Windows Phone to tablets: Not bad, but with the former's(or later's) market share (0% and ~2%) there's no chance to fight against iOS and Android, remember, no matter how good is the thing that you make if the market is used to something else it's almost sure that you will fail unless you bring something different to the table (ask HTC or the high-end ultrabook makers competing against Samsung (the former) and Apple products(both).
B) Do “what actually happened”: At least you force some users and get a baseline for your market share, so developers actually get a market to target.
C) Having both, full Modern UI and full Desktop experiences available: Great for everyone except Microsoft trying to get users to use Modern UI apps. To be honest I believe this one should be the implementation in a single OS release
D) Having 2 branches of the OS separated by version numbers like the Windows 7.1, 7.2, ~, 7.9 Family for the Enterprise and Power User markets and the Windows 8.x Family (Windows 8 as it's right now) for either the Consumer or Enterprise markets: Really, really great and it also gives Windows 8 some freedom to face some problems since you don't have to deal with the Enterprise market but it will be expensive, really expensive. This one is the ideal case
Microsoft is a company if they don't get into the tablet experience thing they will eventually lose relevance, not against Android, but against Apple. Unlike Google, Apple got OS X as a “Workstation ready” OS and in many professional fields (media production for example) it gives a great fight to Windows.
If iOS keeps improving and the devices keep getting more powerful processors Microsoft will lose the users that use applications that aren't too resource intensive (Microsoft's flagship Office Suite for example) on Windows to iOS and then the ones doing really resource-intensive tasks will ask themselves “Well there's Windows and Windows Phone, but there is OS X and iOS for iPad and iPhone, and they have apps that offer integration with this application that happens to be the one I use in my workstation” which one will that people prefer in the long term?
So Microsoft didn't have another option really.
However there's a case for something that Microsoft can add to the desktop and it's a way to get into the administrative tools that isn't as cumbersome as the dialog box that you get when you use the right click in the lower left corner
Disclaimer: This is my personal opinion if someone, in someway, felt hurt, insulted or disrespected by this text I apology for it beforehand.
Also I want to thank everyone that reads this, since it's a pretty long comment.