Emergent phenomena were a big one for me, too. One of my favorite pieces on the topic of economics is Hayek’s: http://www.econlib.org/Library/Essays/hykKnw1.html

Dawkin’s “The Ancestor’s Tale” showed me this pattern in biology, too, and drastically rewrote the way I see the world.

Being able to follow and understand mathematical proofs for the first time was also enlightening. I prefer the simple, clever ones, such as Cantor’s diagonal proof of the uncountability of the real numbers: http://en.wikipedia.org/wiki/Cantor's_diagonal_argument