What to backup in Linux

Every system should have a backup strategy. It is often not clear what needs to be backed up, and what can be safely ignored. The hierarchies of /var, /etc, and /home are valuable and cannot be restored without a backup. Therefore, these are primary candidates for backups. I have never backed up more than:

/etc, /var, /usr/local, /home, /root and /srv 

This is quite enough to have always been able to restore a broken system or a deleted file when the need arose.