If you read the Windows end-user license agreement, instead of clicking without reading, the way that most people do, you will know that you do not own your copy of Vista; you are simply licensed to use it. To continue to use it, you must activate it, and how you use it is severely limited.
You cannot, for example, install it on another computer. Nor do you have any control over any so-called digital rights management (DRM) techniques that might be installed on your computer to do such things as to police the legality of your music downloads. If Microsoft determines that your copy of Vista is not valid, it can disable the copy -- and mistakes have been made about validity.
On a GNU/Linux desktop, none of these problems arise. The free and open source licenses used by the software allow you to use the software on as many computers as you like, without any need to register, validate, or activate it. Because the source code is available to everyone, you can be reasonably sure that no DRM is installed on your computer, even if you are not capable of investigating the situation yourself. At the most, a few programs will ask you to register voluntarily so that those who write them can have some idea of the number of users.
In other words, on Windows, you have no rights in what you purchase, and no control over how you use it. But on GNU/Linux, you have much the same control over how you use the software as you do over a piece of furniture or clothing that you buy.
Here and there, Vista has features that GNU/Linux does not. It is still easier to set up dual monitors in Vista than in GNU/Linux, although that will change in the next releases of the major GNU/Linux desktops. Vista also comes with some basic speech recognition, and has a few controls that GNU/Linux lacks, such as a dialog for changing mouse cursors. While you can have these functions in GNU/Linux, as I write they are not available out of the box.
However, the opposite is also true in GNU/Linux. The ability to roll up a window so that only the title bar remains is a feature that I would miss if I ever needed to use Vista regularly. Much could be said, too, of the fact that GNU/Linux is designed as a secure, multi-user system, and that, while Windows now has features like password protected user-accounts and permissions, they are voluntary and tend to be neglected.
At any rate, these additional details do not change the general picture. Overall, GNU/Linux offers more choice, customization, convenience, control, and consumer protection than Vista does. These days, GNU/Linux not only matches Windows on the desktop, but frequently exceeds it. Moreover, as KDE 4 shows, GNU/Linux developers are starting to innovate on the desktop rather than scrambling to catch up.
These are the reasons why, when people ask whether GNU/Linux will ever be ready for the desktop, I wonder out loud why anyone would ever use Vista or any other version of Windows. No matter what perspective you take, GNU/Linux is simply a more appealing environment in which to do my daily work.