When I ask the customers of my clients what they really care about the same top three points come up almost every time.
1. Your expertise
2. The quality of your product/service
3. Your price positioning
Get these three things right as perceived by the client and they will be happy. But, that is just the cost of entry. To make her/him so happy that they continue buying from you needs digging deeper into other aspects of what they really care about. These additional factors relate to processes, relationship and trust and will be very specific to your company and your customers.
So you have to ask and not assume what makes them happy or unhappy.