For some reason the settings for the /var/tmp directory conflict with a mysql install on TonidoPlug 1, and the root user does not get created. The solution for this is to change the permissions BEFORE installing mysql.
chmod 777 /tmp
But what if you didn't already know this, or forget? You probably know it's pretty irritating trying to completely uninstall or reconfigure mysql easily, and change the root user's password. These instructions are useful for re-installing mysql from scratch. I did
apt-get --purge remove mysql-server
apt-get --purge remove mysql-client
apt-get --purge remove mysql-common
apt-get autoremove
apt-get autoclean
And re-installed with
install mysql-server mysql-client
This then prompts you to enter a root password and all that.