Being that my background is in design and marketing, and coding is an late add, I value every tidbit of expertise I can absorb, and I've absorb a lot from working with Joel and John.
The two primary things I've learned that stick out are really more like mantras I've heard preached over and over and over again...time after time.
"Computers only do what you tell them to."
This is Joel Worrall's mantra. The point is to always remember, there are seldom any real computer glitches. If there's a bug, it's because you wrote it into the code. If something isn't behaving correctly, it's almost always because that's how you programmed it. Computers don't just randomly not do what they are instructed. In fact - that's all they CAN DO: what they are instructed to do.
The practical application for me that I've learned is to check and double check my code for errors before I go complaining that something isn't working right, or being tempted to file a bug report with an application's developer. It's probably not an actual bug, it's an error in the code you wrote.
The other application has been to develop a pattern of thinking when I'm trying to solve problems that asks first, "Okay, what have I told the computer to do here? Or what is it doing based on what it thinks I've told it to do?"
That type of thinking has helped me to troubleshoot more issues than I could buy Joel beers for.
"Did you clear your cache?"
This is JK's mantra. LOLz a bit, but it's so true. Such a simple issue that is often overlooked. Or more accurately, the once or twice you end up overlooking that issue it is most likely going to be the problem.
JK is a very talented developer and this simple advice is really epitomizing of the fact that true talent often boils down to having the discipline to remember and practice very simple axioms.
So in honor of JK, don't forget to clear your cache.