I'm guessing:
The Windows 8 Desktop side will be top-notch, comforting, and work as before and as expected.
The Windows 8 Metro side will be top-notch, internally-coherent, well-received, and offer a reduced complexity set for novice users and those seeking clarity and easier consumption.
The core OS will be top-notch, work on a grand range of hardware, and work as before and as expected.
It's not the problem with any of these pieces, it's how they're put together.
The first hour of Windows 8 is maddening, regardless of your technical skill level, comfort with Windows idioms, or level of resistance to change.
Two thirds of system configuration has been gutted and stranded in Metro mode. Login (if set up) is baffling and new. The computer violently flips between Metro and Desktop for reasons you're never quite sure of. It's constantly trying to drag you to this new place with completely different rules, appearance, and behavior. UI is hidden and transient.
There's no guide. You're just dumped there, and all efforts to work your computer like you did before are stymied. Yes, eventually YOU figure it out, but someone who's spent 20 years baking in the same ultra-reduced set of magic rituals to get the e-mail out is going to feel stupid. And they're not going to blame themselves - the era when you had no choice but to blame yourself is long over.
The Microsoft TABLET will be fine. The Microsoft SERVER will be fine. The Microsoft POWER USER base will be fine. The Microsoft GAMER population will be fine. The Microsoft DEVELOPERS will be fine.
It's the people who are only on the platform by virtue of "that's where facebook and my email is" who are going to run away screaming "Vista 2". That's the chip they're gambling with here, and I think they're going to lose it.