Starting with Fedora 9, the KDE desktop was essentially destroyed (release of KDE 4.0). Usability went to zero; all of my learned work pathways/flows were interrupted, and despite tons of experimentation, I can't redevelop either them or alternatives in KDE.
So I switched to GNOME.
Now with Fedora 13, GNOME is seeing breakage again. They've stopped displaying icons in the icon selector (who's bright idea was it to force users to choose icons by filename, rather than by looking at icons in a grid?) and removed both "Eject" and "Unmount" from device context menus, so that the only option now is "Safely Remove," which spins devices down. Not good if it causes your RAID (as it does mine) to lose at least one drive's sync each time this happens, forcing a rebuild on next use. Of course, neither of these changes… can be changed. The totalitarian devs of both desktops are hell-bent on ensuring that nobody in the primary Linux demographic can actually use Linux any longer.
And of course with Fedora 14, they're doing away with the GNOME desktop altogether. (They say they're not, that they're just introducing "GNOME Shell" with GNOME 3.0, but this is essentially a new desktop with zero UI continuity from the previous desktop, so it doesn't matter.)
Very likely after Fedora 13 becomes obsolete it's Windows or Mac OS for me. Linux is a better system under the hood but if you can't actually use the damned software because of the constant UI Nazism, what good is it?