The backlash came from the fact that it was a change from 20+ years of ingrained behavior, not that it was an inherently poor design. However, you could argue breaking the previous mental model of users will definitely create issues and therefore is a bad design.
But that's just me -- I also didn't like the sliding start menu design either (Windows 7, KDE 4), where submenus open in the same space as the folder icons -- I'd rather, when clicking on a submenu, have it open into new screen real estate like any of the traditional menus.
Is this a serious question?
Also I think the "start menu" argument encompass in reality the whole concept of Metro against traditional desktop UI.
I'm really hoping that this all makes sense once you start using it, as I'm going to have to make a decision on what to upgrade my parents' computer to (Win 7 or Win 8) -- I don't want to confuse them.
Edit: I agree it looks crazy, but at least it behaves consistently!
It's not the exact same thing, but it reminds me of the weird misadventure they made in Office 2000 where they would hide infrequently used menu items. It sounded good in theory, I mean, less menu items should mean less confusion right? But in practice the main response was "where the hell did that thing I was looking for go?" and "why does my menu keep changing?"
As to your second point, I think the question is how do you improve your UI without breaking previous mental models/behavior?
The more things change, eh?
Opening the Windows 8 Start screen loses all the visual context - your taskbar disappears, your windows disappear, there's nothing to tie it into the desktop, it feels like an entirely different system from the one you were using a second ago. And that's before the fact that how you interact with it is entirely different from how you interact with a desktop app, which is the final wedge.
Luckily, they're integrating Metro into the desktop properly now, so the entire system will be usable.