What do you wish you had known before you  started with Microcontrollers?
For me it's the other way around: I wish I knew about Arduino's and AVR's before they tried to teach me electronics in secondary school. Learning Ohms law without ever having seen or used an actual resistor was not a pretty exercise. Back then it would be ideal to have an Arduino around to play around with.

Because of this, I would have like to have known more about basic electronics. Simple questions as: why should I care about current? (because things blow up if you don't) Why should I care about resistors? (same here and other stuff) Why should I care about caps? (all kinds of reasons) 

Having the ability to play with micros (and frying them in the process) has tought me most of these things but I wish I had learned them sooner. (Classic chicken and egg problem I guess)