I had already been a Bayesian and fan of Kahneman-Tversky for 15 years before I started reading OB. So I didn't learn the basics there.

Given that, I'd have to say the most important thing I learned was to exercise unflagging discipline when thinking about values. For me, this means (a) remember to keep track of terminal versus instrumental values in any problem framing and (b) realize that most people's terminal personal values are complex and != "maximize pleasure".