Or from a different angle:
How do your customers measure satisfaction with you?
The basics are quality, cost and delivery, but there are numerous other smaller issues like communication (availability of staff to support enquiries, phone fax and net access to support or ordering, how your staff answer the phone, how long it took them to answer the phone, how helpful are your staff when talking to customers, etc, etc.), knowledge of staff, ease of invoicing/payment, clarity of shipping and packing documentation, clarity of labeling, and on it goes.
SO, back to the original question, what is a customer concern? Anything that has a negative impact on the customer's perception of your company, whether they bring it to your attention or not.