What's the single most important lesson you've learned from playing D&D?
I'm not really sure, I don't like questions like this one personally. I've learned how to be more creative I think. When I run a game I tend to pull things out of nowhere, whether it is random creatures or rooms to whole dungeons being built as the characters are going through it with nothing prepared ahead of time.
I've learned the enjoyment of playing a game with people who have similar interests, and the fact that not all people play the same, so two games doing the same exact thing can be completely different from each other depending on who is playing.