People above in my opinion are describing an "average" player not a "good player."

-A "Good Player" should know not just their job but all three job types (Tank, Healer, DPS). Actually leveled each class type.
-A "Great Player" should know the differences in the jobs, how they interact with each other, and when best to use what set up. (PLD vs WAR; SCH vs AST; DRG vs SAM for example).

-An "Excellent Player" has all the above and is not a dick about it. XD