I've stopped following KDE like 5 years ago, but I remember constant changes to the multimedia APIs. Also each new QT version (2->3->4, now 5) brings incompatibilities, no?
The multimedia API changed from KDE 2 and 3's aRts to KDE 4's Phonon. The API is still Phonon, and this will be true in KDE 5 as well.
There were other application-specific APIs that were used for things that were beyond the capability of aRts and Phonon (e.g. Juk could use gstreamer directly in KDE 3) but the KDE APIs themselves were pretty stable.
It's true that a Qt change in binary incompatible and involves at least some source porting effort, but honestly those have been fairly infrequent, and only one of those transitions (3->4) involved more than minor source changes. Qt 4 was released at the end of 2005 (around the time of Linux 2.6.15) and is still the "current" version, though the Qt 5 release is around the corner.
In just the KDE 3 timeframe to now, on the other hand, there has been gstreamer 0.6, 0.8, 0.10, soon 1.0, PulseAudio, ConsoleKit, HAL, HAL's replacement (DeviceKit), DeviceKit's replacement (udev and friends, upower, udisks, etc.), PolicyKit, PolicyKit's replacement ("polkit"), NetworkManager 0.8, NetworkManager 0.9.
Needless to say, trying to keep in sync with all of that has not been fun. :-/
