When I got my OW certification back in 91, my instructor actually told me NOT to take any more classes. Think about that for a minute. His advice was not to take additional classes. How vastly different is that from what you normally read on SB. His advice to me was to go diving ASAP while all the training he provided was fresh in my mind. So the very next weekend my buddy and I rented some gear and tanks and off to Florida we went to dive the areas of the springs and rivers that was appropriate for OW diving. Soon after I was in the Keys diving with some friends and then started diving off the coast of NC on a regular basis. I became hooked and purchased my own gear and have been diving ever since
I did ask him about other courses but his answer was basically this, "I have taught you skills and given you the basic knowledge you need to dive. What I can't give you is experience. That you have to earn. If you need any additional classes, I didn't do my job." Eventually I decided to get AOW because I was being told some operators want you to have it so I went back to my instructor and he said sure, no problem. He could see why someone would want that cert. We did that and basically just repeated dives I had already been doing. By the time I got my AOW, I had numerous wreck dives, night dives, cavern dives, dives to 100+ ft. etc. with no additonal classes. I believe my instructor was a top notch one. He gave me good fundamentals to build on and I have done that over the years. Now would I go cave diving without the course? Absolutely not. Some areas of diving I don't believe should be self taught and that is one of them although I'm sure that's exactly how the early guys did it before there was a course.
By in large I found recreational scuba diving to be pretty easy to learn and I still believe that to be the case. A good open water instructor and some common sense will take you a long way.