You can't coach genetics. There will never be a coach that can take a kid who is 5'4" 110lbs, and make them 6'4" 235lbs. It's impossible. There's a fortunate group of people who are born more gifted physically and athletically. With that being said -- a "great coach" understands that. A great coach has the ability to recognize the limitations of his athletes and formulate his game plans and offense/defense to complement the strengths his athletes do have, and minimize/hide their weaknesses.
There's plenty of examples of head coaches who have taken jobs nobody wants and completely changed the culture and had great success. There's obviously going to always be a ceiling as to what you can accomplish, but if a coach walks into a program that has 3 wins in the last decade and is able to get 2 wins in a season -- that's a good benchmark for success. Great coaches don't have to be undefeated or win championships to be great - they just need to recognize their weaknesses and limitations and find a way to work with what they're given. Can they lead, can they get everyone to buy in, are they making an impact? Can't just define a great coach by W/L -- there's a lot of different markers for it.