Totally ignoring all the specific changes which are very debatable I think the biggest challenge Microsoft has is a significant number of PC owners are already on the fence about buying another PC. The one they have works OK and they have shifted a lot of their day-to-day computing to SmartPhones. Quite a few of them are already at least casually interested in the iPad instead of a new PC. When they look at Windows 8 they have to decide if it's worth investing the time to learn something new in an era when they are using their PC less than ever. It's kind of the perfect storm with a higher barrier of entry and a lower value proposition to the consumer in the end. Tablets are a different situation though I think it's also a huge uphill battle for different reasons.
The scary part is they have unintentionally interweaved the fate of all their different platforms together. If Windows 8 is a huge flop on the desktop who's going to buy a Windows 8 tablet or a Windows Phone? The Metro UI is unique enough that the taint of failure will stick to it. Metro could become the new Clippy. The mere sight of it could make people recoil in horror. No one could exactly tell you why they hated MS Bob, Clippy or Windows Vista. They just knew you were supposed to hate it because everyone hates it. If that happens with Metro Microsoft is looking at another 2-3 year 'reboot' and I just don't think they have that much slack left to work with.
The scary part is they have unintentionally interweaved the fate of all their different platforms together. If Windows 8 is a huge flop on the desktop who's going to buy a Windows 8 tablet or a Windows Phone? The Metro UI is unique enough that the taint of failure will stick to it. Metro could become the new Clippy. The mere sight of it could make people recoil in horror. No one could exactly tell you why they hated MS Bob, Clippy or Windows Vista. They just knew you were supposed to hate it because everyone hates it. If that happens with Metro Microsoft is looking at another 2-3 year 'reboot' and I just don't think they have that much slack left to work with.