The top three qualities of customer service are:
A genuine desire to help others. All good customer service staff start with a keen desire to help others. The essence of customer service is helping customers to solve their problems.
A wealth of technical knowledge. Wanting to help is not enough. Customer service staff must have sufficient technical knowledge, (or at least be able to access that knowledge) which will solve the problems presented by the customer.
Excellent communication skills. In order to gain information from customers, and in order to explain information to them, customer service staff need to be excellent communicators. Both in spoken and written form.