In short, KDE 4 is about one thing and one thing only: 3D rendered eye candy. If that’s what you want, that’s what you’ll get, in spades. But as a desktop, as a single, integrated, holistic sense of place and set of potentialities and operations that are intuitive, minimal, and streamlined and that support productivity, KDE 4 is an epic fail in a way that makes KDE 3 roll in its grave.
That specific comment states the plain truth about KDE4. I tried to run it on my trusty Thinkpad T60 for a few days, finding out it is basically unusable with compositing turned off. Turning it on makes it work too much for too little return; I couldn’t even listen to music on Grooveshark without the music slowing down!