Why is organic chemistry so challenging?

I have taught this topic for many years now and so, I have pondered this question at length.  I have come to the following conclusions.    

1. The fear and apprehension that students have for the course is real.  Org Chem has clearly left a less than positive impression on students who have taken the class in the past.  It is to the point where I will avoid introducing myself as an organic chemistry professor.  So, in a way, one reason for this website is for my own self-preservation. When I run into former students in the future, hopefully, they will have had a more positive experience and impression of the class.
2. Organic chemistry appears to be one of the first challenging college course for many students.  Often students study as they normally do using the same study methods that have worked in the past but suddenly these methods do not seem to be working in Org Chem.  When this happens, it is such a dumbfounding experience that students do not know what to do because it has never happened before.  It turns out that the material and problems in Org Chem are sufficiently unique and challenging that they require some flexibility and reevaluation of study methods and habits.
3. Organic chemistry is not like other science courses.  Students who have done well in Biology, Physics, and General Chemistry often find that Organic Chemistry is a different beast.  Organic chemistry appears to require a different set of mental and study skills from most science courses.  Good memorization skills or good algebra skills that served you well in Biology and Physics will only get you so far in Organic Chemistry.  Instead, the class requires abstract logic, three dimensional visualization, and rule learning/application skills.  The problem-solving skill set and study methods for organic chemistry are actually much closer to that of a foreign language course than to other science courses.
4. Given the right approach and level of effort, Organic Chemistry is not that hard.  A large number of students do really well in the class and walk away happy with their grades.  Some students even confessed (mostly in private) that they enjoyed the class.  What people liked about it the most was the way all the things that they learned throughout the semester fit together, as opposed to learning random facts and concepts.