Lots of good advice about concrete skills mentioned here already - eg scripting, monitoring, documenting, low level OS knowledge etc. I reckon a good sysadmin

Lots of good advice about concrete skills mentioned here already - eg scripting, monitoring, documenting, low level OS knowledge etc. I reckon a good sysadmin also has a hint of nagging paranoia or slight sense of impending doom and is never complacent about how well things are currently running or how secure they seem. On top of this they need a good instinct for anticipating problems and evaluating risks so they can proactively fix problems before they arise. Oh yeah - learn how DNS really works.