By far, the most visible new "feature" in Windows 8, is it's new UI, which takes inspiration from smart phone and tablet devices. The old start menu is now full-screened, with large icons for all apps, and apps run in full-screen by default, changing a desktop PC into a very large tablet minus touchscreen with a keyboard and mouse added on.
It's not surprising in the least that many users take issue with this. Early on, people have said something along the lines of, "Oh it's just for the early builds, surely they will allow some way for long-time users to disable it." However, now it would seem that that would be only wishful thinking, at least for the time-being.
This is a sharp turn for Microsoft from their previous UIs. Aero, found in both Windows Vista and Windows 7, allowed users to disable it if they didn't agree with it's aesthetic, or wanted to reallocate the memory from the UI to applications. Moreover, Aero was still functionally the same as older Windows UIs. It may look prettier, but it still fires up a Start Menu like before, still lets one dock things into the taskbar, and still lets the desktop get cluttered up with icons.
It's this difference that's key here. For companies that have Windows deployments with hundreds or thousands of seats, changing the way a Windows UI works is not an option. Regardless of how easy to use the Windows 8 UI may be, it's still not the same as what users have been trained to use since 1995. Sure, Windows 7 isn't Windows 95, but changes have been introduced gradually over time, making new features easier to adjust to. The Windows 8 UI is a fast, jarring change, that is likely to frustrate users as they adjust. With no clear path to turn it off as there is with Aero, it also makes it more likely that administrators around the world are less apt to adopt Windows 8 quickly. After the debacle around initial releases of Windows Vista, one might think that Microsoft had learned their lesson. Even Microsoft wasn't too popular to make an OS that no one wanted, and Windows XP lived on far longer than anyone ever thought it would. Windows 8 has already suffered from its share of bad press even before the official release. The logical thing to do here would be to be proactive in heading off user complaints.
That's why it's rather surprising to see them take a hard stance on the Windows 8 UI. Sure, undoubtedly some third party will create a drop-in shell replacement eventually. That's been done in past versions and will likely be done again for Windows 8. For a home user, it's an acceptable path. Home users of Windows are used to beating it into submission. However, for any company that has deployed hundreds of Windows seats, mandating the use of a third party shell replacement just isn't an option, much like Windows 8 isn't an option at present.
Short of opening the source to Windows, it's reconfigurability has, until now, been rather accommodating for users. Through the use of registry settings, or third party software, users have been able to configure Windows for themselves until they feel it's sufficiently usable. While still not "free" in the GNU sense, the UI has still allowed users this semblance of freedom, to do with the UI as they will. Since a normal user wouldn't hack at the source anyway, giving them the tiny bit of freedom to determine how they interact with their UI is what keeps them as a user. What Windows 8 is looking at here, is backlash not unlike the transition from GNOME 2 to GNOME 3, albeit on a much grander scale.
What will be the final outcome? That's hard to say at this point, as Microsoft could still change their stance and implement a way to bypass the Windows 8 GUI and bring up the legacy desktop. As it is, there are several keyboard shortcuts that allow this, it's just not possible to do so automatically at boot, which would seem to be what legacy users would want most. There's also an opportunity here. If people with large Windows deployments are faced with having to retrain their users, they may think about training them on Macs or Ubuntu or something else instead. The most likely scenario though, is likely the one that we saw with the release of Windows Vista, and that is that Windows 8's predecessor will be around for a lot longer than Microsoft planned.