PADI's prerequisites for its Divemaster training are a bit thin. All you need is 40 dives to begin, and 60 dives by completion, along with the other requirements. I completed 100+ dives before beginning, another 100+ dives during my training, and after seven weeks I still feel as though I could've benefited from another week or two underwater, especially with regard to navigation.
I learned to pay attention to my instructors, sure; but I also learned to do your own research. On more than one occasion I heard instructors, and a majority of them, provide information that both PADI and DAN either no longer recommend, or disagree with entirely (e.g. turning back the tank valve a quarter-turn, facing the SPG away from you when you turn it on). When I questioned some of the discrepancies, the general answer was that the advice couldn't hurt (specifically with regard to turning back the tank valve a quarter-turn); however, I'm of the opinion that unncessary information just distracts from the necessary information. Keep things simple and streamlined, not just with your equipment but with the information you provide.
I also learned that the best approach is to develop your own style. Take what you like from certain instructors, and ignore other things. For example, I found most dive briefings to be too thin, and lacking emphasis on personal responsibility underwater. There is a fine line between babysitting and broad planning, and a Divemaster's role is unquestionably the latter.
Sadly, I also learned that not all dive professionals respect underwater life at all times. Too many times I saw dive professionals touching marine life. Now that my training is complete, I'll have more of an opportunity to be a role model and not just a student. I'll have a bit more of a voice, albeit not as much as an instructor, in most cases, which is why despite my lackluster enthusiasm for instruction I'll probably consider getting certified as an instructor, if for no other reason than standing within the dive community. Like most things in life, there is a hierarchy.
I learned that the benefits of diving trim are incredible, and that after having gone to a backplate and wing, I'd never again dive regularly with a BCD. It's just not for me.
I learned that there is no replacement for being prepared. Diving with a backplate and wing allows you to strap more gear onto your kit, which at times might raise eyebrows. Do you really need a knife, slate, whistle, flashlight, and emergency strobe on every dive? Of course not, but if you don't bring them on every dive, you won't have them when you need them. Instructors routinely asked to borrow my equipment underwater, because they themselves weren't prepared.
Finally, I learned that there is still so much that I don't know, and still so many experiences to be had. I've dealt with panicked divers underwater, and rescued a diver from a capsized boat (only days after completing my Rescue course), but despite having managed these things successfully, there are no guarantees in these situations. With each experience, you improve. The area where I feel I need the most improvement is navigation. Some of it is confidence with a compass, and some of it is just learning the reef, but I know for sure that I'm not where I could be, or even should be.