Which Folders To Include In backup?

I'm quite new to Ubuntu and want to create a backup. I'm really not sure what files and folders to include so that if I restore my system it will be as it is now. I can't seem to find good details of this anywhere. Hopefully someone could help me with this. Is it possible to backup everything as it is now so in the event of a system restore I don't have to reinstall programs and settings?

My backup sets currently contain

/etc
/home
/root
/srv
/usr (/usr/local only, nothing else)
/var (except /var/run, /var/cache, /var/tmp)

Note this is for a server, so backing up things like /etc saves all my configuration for my services, I have web servers in /srv (though if you have them in /var/www, they would still be in this backup set), I have various scripts and things set up in /usr/local, etc. Backing up /home instead of /home/myusername is so that I can preserve all users. If all you want to keep is your user data, you only need your home directory.

Edit: If you have anything in /opt you probably should include that too. Thanks to @GabrielStaples for pointing that out.