GNOME, KDE and Windows desktops were great when they were in catch-up mode (with Mac OS).
Windows peaked at Windows 98 SE; every change since then has been negative; Windows 8 is its death rattle.
KDE peaked with v3.5; I haven't been a regular user of GNOME so I don't know when the rot set in, but it is not the highly usable system that it once was.
I no longer migrate non-technical friends to Linux; I recommend Windows 7 in "Classic" mode, which will not reach its EOL until 2020. For techies I still recommend KDE 4, which I use myself, but I have given up on kmail, which committed suicide when it gave up maildirs and switched to the temperamental Akonadi backend. Please! How do you explain to someone that they need to restart a database before they can read their email?
It is not that all the innovations have been bad; it is that, when a system is close to perfection, most changes will be downhill; and while amateurs can code as well as the professionals, the creative skill needed to imagine a new yet workable GUI/desktop paradigm is exceptionally rare. Therefore, many projects reach feature-completeness, and then commit suicide because their developers feel the need to innovate.