Why do you have to open the pneumatic crossfeed valves to de-ice the wings? Exactly what does the AC bus cross-tie switch do? And why do the engines only draw fuel from the center tank when all the fuel booster pumps are on? The key to understanding these and many other aspects of your aircraft is studying the schematic drawings for the various aircraft systems. That may sound a little bit dry, but to help you out I have included a Schematics section in the DC-9 Classic Flight Center that gives you live interactive schematic drawings.
Radio navigation is the name of the game when you are flying a DC-9. Radio navigation, like all new things in life, can be difficult to understand at first. But once you have mastered it, it becomes second nature and you will have a much better mental picture of where you are, where you are going, and how to get there.
Failure simulation. This is what in my opinion takes the DC-9 Classic, and all other addons that includes such a feature, to the next level. Usually when you simulate a system in an aircraft, you simulate ON or OFF in a normal state. With failure simulation a system can also be simulated ON or OFF in a failed state. While OFF is usually OFF regardless of state, ON and failed at the same time is where it can get really busy real fast on the flight deck.