I was reading about AdS/CFT and its possible description of superconductors. It is said that superconductors may exhibit a strong coupling behavior. In general, AdS/CFT is thought to be a good tool to study strong coupling dynamics. Now, my question is what are the properties that define a strongly coupled system? In my mind QCD is strongly coupled because of the confinement of quarks and gluons. But it seems that this may not be the case. Can anybody explain?