Given that C++ libraries are getting so complex (now with move semantics!) there is no way a library user can understand why something does not work as expected. Especially when dealing with sophisticated libraries: Boost, Eigen, etc.
As far as I understood it, this was merely due to discussions what types of Concepts would be supported. There was one group of people that wanted very granular Concepts, and another group of people that wanted just very broadly defined Concepts. They never reached an agreement on this.