I've been a KDE user for many years, but with the advent of version 4.0, I was forced to switch to Gnome. Why?
1. The default KDE looks like Vista. I find Vista's GUI to be an ugly, unpleasant monstrosity. What's with all this black, anyway?
2. KDE 4.0 was buggy and incomplete. If I wanted to use beta-quality software as Release Quality, I'd still be using Windows.
Hopefully, by the time the developers get around to screwing up Gnome with some new "artsy-fartsy" new look and feel, KDE will have gone through enough iterations to be feature complete and stable (not to mention have plenty of new non-black themes I can live with).