"Be careful here, though. Even seemingly trivial things like UI or VFX can have a massive impact on the usability of your game."
UI has a massive impact on usability? Well I'll be darned.
In most software, the UI is what you use primarily to interact with the software. In games, what's called the "UI" is usually what you use for tasks like saving and loading games, and setting options. It's also frequently given to a junior programmer to implement.
I'm a professional game developer, and I think the article makes some really good points.