A guy at work bragged he was about to finish up his MD certification. I asked him how many doubles dives, deco dives, wreck or cave penetration dives, trimix dives, rebreather dives, etc. he had. The more I asked, the more I got the "deer in the headlights" look. When the final answer was 1 (for the time he poked his head into a wreck), I asked if someone at the top of the game, a MASTER, should have some experience with those?
I can think of two non-fluff PADI speciality courses, Nitrox and Equipment Specialist and two that are almost non-fluff if you really need the help: PPB and Drysuit.