I like the list of OS you give, but System Administration is not the configuration of a system but instead thousands, with a mind towards performance and allocation of resources, but also includes stressing systems to see what will break. What is being administered is actually people, not machines, and perhaps ironically, relationships become of fundamental importance. Generally, sysadmins are not software developers; they are technology generalists and experts at any operating system and every application, even those they have never used.