KDE 3.x to KDE 4.x
'Ubuntu's drive to bring something radical and new may end up creating another KDE 4 situation - the initial release was clearly not ready for prime time, but now that KDE has matured few would opt to go back to KDE 3.x.'
KDE 3.x was bliss to use, easy interface, almost perfect.
KDE users departed KDE 4.x after KDE 3.x was dropped in large numbers and still show little signs of having returned.
More seriously, I dropped Mandrake over their 2 disc installation problems around 2003 sometime, switched to Suse then had to drop Suse sometime around 10.0 because of the inability to do something simple like install software - another case of changing something that worked perfectly well and replacing it with something 'new'. Finally I switched to Ubuntu in 2008 after sticking with an older version of Suse for as long as possible. Now it sounds like Ubuntu have dropped the ball like Mandrake and then Suse.
Having finally got used to Gnome, now it is being dropped by Ubuntu.
Still I least I have Ubuntu 10.04 LTS installed which is good for 3 years of support I believe, which puts off the decision for 2 more years...