I was going to ask the same question so I just decided to post here instead. My top 6 lessons:
Patience – Nothing good happens quickly in your aquarium and this is very true for life in general. Regular thought-out and persistent work usually equals a reward in the end.
Health – Seeing how amazingly my animals responded to dosing and supplementing elements, minerals, and vitamins to my tank got me to focus on making sure my own intake of vitamins, minerals, and healthy food was optimal to manage my own health.
Mutually beneficial relationships – Observing the symbiotic relationship between zooxanthella and our corals, a pistol shrimp and goby, cleaner shrimp and fish, clownfish and anemones taught me that to succeed in life, business, and relationships you must create mutually beneficial relationships.
Balance - Too much of a good thing? Too much light, too much flow, too high of a nutrient level or too low of a nutrient level. It all comes to balance in a reef tank. This taught me to stop spending too much time and being obsessive with my wonderful reef hobby or even working too much. There are also many other wonderful things to spend time on like cooking, gardening, traveling, nature, cars, watches, and especially family time.
Resilience – Seeing sick fish and corals come back to life after addressing disease or environmental problems. This has taught me to not give up no matter how bad the situation looks. As soon as you do you have already lost.
Listen – Thinking you have a magical power or better luck to avoid problems than anyone else is a big mistake. Listen to those that have experience and follow advice.