Sources: Microsoft , [2 via Twitter]
quote: In Windows 8 we assume that there are even more apps (and sites) than the XP/Vista/7 eras and so we needed even more scale. We also wanted to provide an at-a-glance view and a navigation model that requires much less dexterity. By using the full screen, we can now show more apps without the need to scroll or navigate hierarchy. By flattening the hierarchy, we provide a way for you to leverage the iconography of the apps and remove the burden of clicking through folders trying to find an app under its manufacturer’s name. Over time this will also address another common complaint, which is that when renaming, combining, or reorganizing folders (which you might do in order to keep the menu from wrapping) you would lose the ability to uninstall cleanly, and thus subject yourself to a periodic garbage collection of your Start menu to avoid dead links.
quote: ... the current Start menu is primarily used for launching infrequently used apps, while users continue to launch more frequently used apps from the taskbar and Explorer. In fact, 88% of app launches are from outside the Start menu today. Instead, most launches are from the taskbar (41%) and the remaining are split between Explorer and the desktop (47%). So it was clear to us that the Start menu was trending away from being useful and we had an opportunity to redesign it to make it more useful and valuable. We want to be careful in this dialog of spending a lot of energy debating what amounts to a “long tail” usage case.