The difference with KDE 4 was that they had to rewrite it to take advantage of Qt 4 technologies among other things. What we have now is different but not radically so. Gnome and Unity are and it seems that it's just for the sake of change.
I didn't like either one of them. I felt like I was 6 inches tall using a smart phone with a mouse. My monitor is pretty big. They might work on netbooks but I'll keep my KDE either way.