1.5. Know Your Users

Although some people bristle at the term "users" (perhaps due to some system administrators' use of the term in a derogatory manner), it is used here with no such connotation implied. Users are those people that use the systems and resources for which you are responsible -- no more, and no less. As such, they are central to your ability to successfully administer your systems; without understanding your users, how can you understand the system resources they require?
For example, consider a bank teller. A bank teller uses a strictly-defined set of applications and requires little in the way of system resources. A software engineer, on the other hand, may use many different applications and always welcomes more system resources (for faster build times). Two entirely different users with two entirely different needs.
Make sure you learn as much about your users as you can.