The 4.x series was the one where the ideas still making Plasma so powerful were seeded. While I loved KDE 3 the design for 4 seemed revolutionary. Too bad it was alpha/beta quality up until the 4.6 release years later. I fared through all the bugs, crashes and performance issues with a young student's determination (while running Fluxbox on the side) but I can very well understand people doing serious work had limited patience for the issues.
Anyhow, happy anniversary from a long-time KDE user!
It's a story that's very similar to Windows Vista. It gets a lot of hate for breaking stuff at the time it was originally released, but the reason it broke stuff is because it put in a new foundation for later releases to build upon. The later releases were better because of the breakage. Something something gotta breaks some eggs to make an omelette something something.
Anyhow, happy anniversary from a long-time KDE user!