What's in a Button?

A while back I posted this spicily-phrased take:

I appreciate that someone else understands that being a GUI has some basic requirements and “draws to the screen” is not the interesting one. The bar is about 20cm off the floor but everyone forgets to jump.

Despite mostly not working on GUIs in my career, I have strong opinions about consistency and about affordances for beginners and power users alike. So today, let’s take a simple case study: a button.