Judging by this article the tipping point was Office XP with Windows XP SP1, after that the scales really started tipping down. Adding more code for security to patch holes that should not have existed to begin with. More code added to Office, bigger memory requirements. When Vista was released the hardware requirements were drastically increased by 25% more that previous versions. Seven years is a long time to update an OS. Windows is incredibly bloated. In seven years KDE has updated itself to KDE3 and release five major update, GNOME has pushed itself up to 2.18 and the kernel itself has put out a major update 2.6 and release 20 updates. Mandriva has gone from Mandrake 10 to Mandriva 2008.1 with 7 updates inbetween. Even Debian with it's long dev/testing cycle has managed to release two versions, Sarge and Etch.
What Microsoft added to Vista? DRM, Aero, security, search? Not all that much when you look at it seriously. Apple has consistently evolved their OS. OSX 10 through 10.4; four major updates, increased security, better features; all without increasing the load on the computer itself. I can't say much about it since I don't use a Mac. Linux is releasing new/update version on a more or less six month release schedule. New features added since 2001: udev, compiz, journalling file systems, virtualization, easy to use package management, security features; innovation is coming from the opensource community at an ever increasing pace. Looking at distros from 2001 and today are like night and day.