I don't want to rip on KDE too much or start any sort of DE war, so I'll just say what doesn't work for me.
1. I can't stand the look of Qt applications. I know it's completely aesthetic and all but still.
2. It's 2012 and I refuse to go grubbing around file launchers and menus with a mouse. I've gotten far too accustomed to using tools like dmenu and Gnome-Do and like to stay as far away from the mouse as possible.
3. While configurability is awesome, there's a lot to be said for sane defaults and not littering every single app window with far too many config options that should really be concealed.
4. While it is possible to handle most of the window management tasks with keybindings, I found only a bare minimum were even set by default, and the ones that were, were not exactly easy to remember. I would have had to reconfigure everything from scratch on every single computer I use it on. I use a LOT of systems so a sane default and less initial config to get productive is very good.