It means that we don't have to support all versions of GStreamer since 4.0 (or stick with the very old ones, which had their fair share of problems), we can transparantly upgrade. That's something very important for distributions, and in turn for users. If apps would not benefit from a new version of GStreamer, the GStreamer team likely would've stopped working on it a long time ago. Old version don't suddenly go bad, they just get left behind by everything else moving on.
Your assumptions that KDE4 applications generally (paraphrasing) have less features than their KDE3 counter parts is a fallacy. We've made the UI smarter in many places, so they *look* less crowded. Most applications are *much* more powerful then their KDE3 counterparts. You'll surely find the odd app that has removed a feature, and often that happened for very good reasons. Overall, however, newer apps are easier to use, in the vast majority of cases more powerful and on top of that more integrated with the rest of the system (DBus, other standards we adopted with KDE4) and better looking.