Windows 8 is a rebirth, a "reimagining" of Windows and the entire
Microsoft brand. It's also the single riskiest project that Microsoft
has ever embarked upon — a bet from Redmond that users can adapt to a
new way of computing. With mobile alternatives from Google and Apple
eating into sales of traditional PCs, Microsoft needed to act quickly to
protect its Windows revenue.
http://www.theverge.com/2012/10/23/3537710/windows-8-review