This came up in discussion recently. Here's how I differentiate between a junior and senior sysadmin:
A senior person understands the internal workings of the systems he/she administers and debugs issues from a place of science, not guessing or rote memorization.
A senior person has enough experience to know a problem's solution because he or she has seen and fixed it before (but is smart enough to check that assumption since superficial symptoms can be deceiving).
A senior person automates their way out of problems rather than "working harder". They automate themselves out of a job constantly so they can be re-assigned to more interesting projects.
But most importantly...
A senior person demonstrates technical leadership by creating the processes that other people can follow, thereby enabling delegation and multiplying their effectiveness. Maybe the senior person is the only one technical enough to work out the procedure for replacing a bad disk on a server, but they document it in a way that less experience people can do the task. Maybe the senior person is the only one technical enough to set up a massive monitoring system, but they document how to add new devices so that everyone can add to what is monitored. Therefore they multiply their effectiveness because they use their knowledge not to do work, but to make it possible that an army of people can do the work instead. Good documentation is the first step to automating a process, so by working out the process, they start the "guess work -> repeatable -> automated" life-cycle that repetitive tasks should follow.
The old way is to maintain your "status" by hoarding information. You are the only person that knows how to do things and that is your power base. The new way is you maintain your "status" by sharing information. Everyone looks up to you because it is your documentation that taught them how to do their job. As they learn by following documentation that you wrote they get better at what they do and soon they are senior too. However now you are the senior person that helped everyone get to where they are today. In terms of corporate status, there is nothing better than that!