MS wanted to create a single, seamless user experience across three distinct platforms: The desktop, tablets, and phones. They eventually wanted to extend that to their web-based interfaces as well (hotmail, bing, etc.). Anyone that tried to tell them this was a bad idea was shown to the door (or had to duck a few chairs).
Then they looked at Apple's OS's, couldn't really understand why people loved them so much (well, some people) and took home the wrong conclusions. They came up with the Metro user interface: all elements are buttons (except the identical looking ones that aren't), and no extraneous, distracting, information is on the screen (also, no additional useful info). Options you didn't need were hidden (and you had no way to find out where they were). Adequate for phones with limited space and limited uses.
After the beta release, they started to see the problems (with desktop systems), but the internal MS politics wouldn't let them change directions in any massive fashion (saying the boss was wrong -> flying chairs). They put the desktop back, but you had to hunt for it. The start menu still didn't exist (except for a third-party add-on (that this was one of the most downloaded apps for Win8 shows how much people wanted it)). Apps were either for the desktop, or for the new Metro interface (which they are now trying to claim they never called "The Metro Interface", an that the OS wasn't specifically designed to use it), and would automatically switch between them when you started one (even inadvertently).
They still failed to understand why people were having trouble: everyone they sat down and taught to use it had no problem (with the things they'd been taught). It worked great with touchscreens. They kept saying "This looks just like Apple's OS's! Why won't people love us?". So several of these problems were still present when they actually released MS Win8.
Of course, many MS fanboys kept saying how there weren't any problems. When asked, though (often repeatedly), they eventually admit that they bypassed metro, installed the third-party Start menu app, and generally had to do a lot of 'fixing' to make it useable (a sub-variant of the fanboy will then chide you for 'trying to use the tablet options on a desktop', heedless of the default installation options). These are all things that the average user wouldn't even know was possible to do, let alone know how to do.
