Originally Posted by chrism01
As lutsp said, root login from GUI is disabled for a (security) reason. It's not recommended to enable it.
You can do anything you need at the GUI by selecting the relevant menu option; if you need root, it'll ask for the root passwd.
Isn't it ironic that people who really, really shouldn't grant themselves root login authority (as evidenced by their inability to find their way around a typical Linux installation) always want it, and a few years later those same people, more experienced and better able to manage the risk, don't want it any more?