System administration is mostly a matter of balancing available resources against the people and programs that use those resources. Therefore, your career as a system administrator will be a short and stress-filled one unless you fully understand the resources you have at your disposal.
Some of the resources are ones that seem pretty obvious:
System resources, such as available processing power, memory, and disk space
Available money in the IT budget
But some may not be so obvious:
The services of operations personnel, other system administrators, or even an administrative assistant
Time (often of critical importance when the time involves things such as the amount of time during which system backups may take place)
Knowledge (whether it is stored in books, system documentation, or the brain of a person that has worked at the company for the past twenty years)
It is important to note is that it is highly valuable to take a complete inventory of those resources available to you and to keep it current -- a lack of "situational awareness" when it comes to available resources can often be worse than no awareness at all.