I realise Microsoft has a huge user base already installed, the majority of the computing World basically. However, the difference between Mac and PC is that Mac users KNOW how to use Macs, whereas PCs are used by everyone ranging from IT proffesionals to computer illiterate ppl globally. Windows is already cluttered, slow and painfully difficult to use for people who are not IT savvy.
However, to drastically change Windows would alienate the people who struggle to get a grip of what they already know because it shifts the goal posts further. That said, it is hard for Microsoft to do anything REALLY amazing.
On the other hand, they've done what they do every single time. Copied Apple. And then they have the nerve to call their anti-monopoly campaign "the freedom to innovate". Please, Longhorn is going to suck. Not as much as XP, but it will still suck.