The meaning of being Great

After much persuasion from a "Mr Blogger" in my organisation, I have decided to take the first "painful" step to creating a blog; unaware of the implications of such a move...I haven't really decided what I want to "create" here, if anything at all. But for a start, I have always wondered what makes someone "great"? Like in a book I read, the protagonist is constantly grappling with the question of "what makes a great butler"? As a librarian, I am often thinking "what makes a great librarian?" Or for that matter, what makes a great teacher, policemen, nurse, designer, etc? Your thoughts?