Today we live in a fast paced society that requires of us responses to stimuli at a faster rate. Most decisions are taken based on past experiences and on prejudices. So it is the purpose of education to help us take steps based on information and knowledge that produces good results. The knowledge acquired during formal education is what in professional and trades constitute the context in which the individual performance is evaluated and rewarded. It is in these arenas that the development of so called “Difficult Concepts” is encountered.
In my experience the first problem we have is the definition of these difficult concepts. What constitutes the difficulty of a concept. Is it the nomenclature? The complexity? The simplicity? Or the depth of thought?
It is clear to me that we have to start with looking at the level of difficulty from the perspective of the level of the individual. What is the previous knowledge that the individual has when presented for the first time with the new concept? Is a question that has to be asked. What is the motivation to learn? As this motivation will give the individual the gumption to invest effort and time in the studying process. Finally, I think, there are external factors like the learning environment, and cultural bias that impede the learning of these concepts making them appear as difficult to the learner.
So the question is how can we identify which concepts are difficult, and evaluate the degree of difficulty. I have the feeling that answering this question will lead us into the development of analytical and critical thinking skills.
David J Terrell 
Good questions and thoughts here about promoting analytical and critical thinking. What does constitute difficulty of a concept? Maybe, to add to what you said, the degree to which the concept differs from our often unconscious beliefs or assumptions? Or the need to hold at once two opposing but equally true ideas?