The thing that bothers me the most about Windows 8 is that Microsoft didn't include the Metro UI because they thought it was better than the old UI.
Everyone can point at an OS which changed its UI in a way they don't like. The thing is, those changes usually happen because the developers genuinely believe that the new UI is better than the old one. Sometimes the developers are right, sometimes they're not. They might make a mistake, but they're trying to improve their product.
Windows 8's Metro UI, on the other hand, isn't there because anyone at Microsoft thought Windows 8 users would like it. That's what bugs me. It's there to build familiarity with that UI, in the hopes that people will go out and buy Windows phones. That's why you can't just turn it off - Microsoft management wants Metro in your face so you'll then go and buy a phone or tablet with that familiar UI that you already know how to use.
It's about using dominance over one market to elbow their way into a different market.