What is the difference between good and great customer service?

Good customer service is a baseline. It indicates that you've satisfied the customer's expectations, giving them the value they perceive your product is worth. An example would be a butcher that doesn't cheat his customers, charging fair market value for his meat and not shorting the weight.

Great customer service goes a step beyond, giving the customer greater value than he was expecting for your good or service. To go back to the butcher, a great one would take the time to explain the different cuts of meat, recommend the freshest product (rather than trying to unload food nearing its sell-by date), and perhaps suggest side dishes or wines that would complement the meal.