Newsforge has an article about new features and changes in KDE 4. An interesting read, but too much marketing hype at certain places.
Qt4 brings a number of interesting features, but is not a revolution. I don't think KDE4 will be, either. Looking back at the development of KDE 3 series, however, I expect the new features will make it even nicer. I hope it wouldn't result in performance problems (it shouldn't, looking at the changes and Qt 4).