The most common problem that occurs, with the Mythbuntu desktop, is the disappearance of the xfce-panel. What is it, you ask? Why, its simply the application that shows the taskbar at the top/bottom of the desktop. If this application is turned off or if it fails to launch, you won't see the taskbar.
To cure the problem, it is best to reset your xfce-panel settings to their original values. The easiest way to do this (you can monkey with the panel settings application through the display manager for a couple or six hours but this usually proves to be fruitless, not to mention frustrating) is to shut down the display manager and then fix up the config files, like so:
sudo /etc/init.d/gdm stop (earlier systems like Mythbuntu 9.04)
or
sudo stop lightdm (later systems like Mythbuntu 12.04)
Once the display manager stops, edit its settings config file for the local user that is being logged on (e.g. "mythtv"). Make sure that the SaveOnExit line looks like this:
/home/mythtv/.config/xfce4/xfconf/xfce-perchannel-xml/xfce4-session.xml:
.
<property name="SaveOnExit" type="empty"/>
.
.
.
Then, get rid of any saved xfce save sessions in the user's cache. For the "mythtv" user, you'd do something like this:
sudo rm -f /home/mythtv/.cache/sessions/xfce4-session*
Once that is done, you can start up the display manager and you should see the panel the way it used to be (i.e. with the task bar):
sudo /etc/init.d/gdm start (earlier systems like Mythbuntu 9.04)
or
sudo start lightdm (later systems like Mythbuntu 12.04)