In some ways, the NewsFactor.com linked to above is a rehash of the reasons for Vista's delay that we've all seen over and over. However, this story also blames the delay on a big part of Microsoft's corporate philosophy and methodology; the insistance on backward compatibility.
Noting the size of the Windows/Vista code base, the bundling of applications that increases that and the perceived need for backward compatibility, the authors theorize that turning such a huge ship (and the gigantic ecology of Windows computing) simply requires too much for the company to deliver on any realistic schedule - period! The authors note that since the debut of Windows XP, Apple has managed to pump out 4 major revisions of their operating system and with compelling innovative features every time. They argue Apple's smaller size, and the smaller size of their code base, not to mention that at every release they have abandoned some backward compatible functions, shows that Microsoft, simply by being so big, cannot effectively innovate any more.
It's worth a read, so you can judge for yourself.