It's not exactly that the new stuff is even that bad - I do use some parts of it. It's that there's a really awkward shift in philosophy that the new UI engages with:

Old: Expose the hardware in a highly compatible way for developers. Help the user do productive work with their applications.

Old basically remains in the same paradigm through WinXP. But starting around Vista the concept changes, and that accelerates with Win8.

New: Help the user shop and consume content. Protect them from malicious software.

And this consumer focus leads in turn towards aesthetics and "wow factor". But it's Vista's shift towards heavy security prompting that I see as the precipitating element in this. It's the change from "they know what they're doing" to "they don't know what they're doing", and the attempt to continually reinvent the same thing but "easier." The UI changes are more like a reflection of the broader trend of aping whatever smartphones do.

And one thing I will credit modern Windows for is this - running on contemporary hardware it's really stable and doesn't blow up easily. But it's like running two operating systems at once, and the newer one is mostly unnecessary.