GNOME 2.0 was released in 2002.
Since then, what significant part of the Linux stack below GNOME hasn't been improved to a degree comparable to what has been improved in Vista or OS X?
Desktops can be defined with huge sweeping changes, but these often lead to delays and removals (see: Microsoft's Cairo.) By making a lot of little changes to the desktop, GNOME has changed as much in the last few years as have Vista or OS X. It's just hard to notice, because you haven't used GNOME 2.0 since 2.2 came out.