A thread over at The Masters’ Council [1] asks What were your defining moments as a GM? [2] — and ties that into the larger question, “How do GMs learn to GM?”
See also: How Did You Learn to GM? [3], How Should GMs Learn to GM? [4] and GM Workouts: Gimme Some Reps! [5] — as well as GMing from scratch [6] and How do you teach roleplaying to beginners [7] in the GMing Q&A Forum [8].