The root account resides on the root partition which is deliberately kept lean and mean to improve the chances of recovery if anything breaks in the system. This means that the root account can easily run out of disk space when running greedy X software.
Monsters such as KDE and Gnome are not suitable for running from the root account. In my opinion it is not a good idea to run any X-windows manager from the root account i.e. no startx or xdm logins from root.
If you want the convenience of X from a privileged account then run X and KDE or Gnome under a normal user with membership of the 'wheel' group and then 'su' to root.