Redesigns of a heavily-used UI are always "usability disasters". People spend many hours perfecting their workflow in one environment and FLIP THEIR LIDS when someone changes it. It's not a rational thing (nor, frankly, an unjustified one). What surprises me is that we haven't figured this out yet and rehash the same flame war every time.
The desktop environments show this best: c.f. Gnome 3. Also Gnome 2. And Gnome 1 for that matter (vs. KDE2). And KDE4. And KDE3. And Windows Vista. And OS X Lion.
The desktop environments show this best: c.f. Gnome 3. Also Gnome 2. And Gnome 1 for that matter (vs. KDE2). And KDE4. And KDE3. And Windows Vista. And OS X Lion.