Like what? (This thread is really piquing my interest.)
Sure... let's see...
Mostly things about major construction methods (metalworking, tube and fabric, fiberglass/composite), what I personally like and dislike about each one, and some surprising facts and misconceptions (in engineering, historic, and practical senses) about each one. A lot of "arcane" details about systems integration, manufacturing techniques, and just putting the frickin thing together. One or two hard lessons about life (I bought my kit and received most of it from a major and up-to-then reputable company less than a year before they went bankrupt... oops, learn, move on, better luck next time...). Which goodies you really want to spend money on--cost/benefit and compromise--or not. And the most important lesson of all if you build your own plane- throwing away some piece that you worked really hard on and starting over on it because the first one is no good. After the first time it gets easier.*

Basically a lot stuff you already know somehow on some level but just don't realize it. Hope all that makes sense!
*Squorch has seen my stack of "bad" fuel tanks...