1) You haven't learned from a mistake until your behavior actually changes in a measurable way. For too long I would think rationally about my life but was missing the important next step: action. (Meta: after this realization a while back I've been tracking 'mistakes' I make and the conclusions I can make from them and have found the exercise very useful).

2) Something more directly LW related: For most of my life I had thought we were beyond the reach of God, but not until I read Eliezer's thoughts on the matter did it really 'click' with me how far reaching and terrifying of an idea it really is. I guess what really clicked was how much the idea that 'everything will work out eventually' is leftover in my thinking from childhood religion and movies where the virtuous prevail.

3) That even the most scientific and rational practices can very easily become empty rituals.