Brett pointed to The Two Things, a page that can be best summed up by quoting:

A few years ago, I was chatting with a stranger in a bar. When I told him I was an economist, he said, “Ah. So… what are the Two Things about economics?”

“Huh?” I cleverly replied.

“You know, the Two Things. For every subject, there are really only two things you really need to know. Everything else is the application of those two things, or just not important.”

“Oh,” I said. “Okay, here are the Two Things about economics. One: Incentives matter. Two: There’s no such thing as a free lunch.”

Ever since that evening, I’ve been playing the Two Things game.