an underappreciated game design skill is the ability to just... sit and think through all the implications of a system by simulating the entire game in your head. you can't just think through the consequences of a change; you need to think through the consequences of those consequences
for a professional game designer1 this is the number one skill you have to cultivate.
The second most important skill as a game designer is communication. If people don't trust and believe in your design decisions, they aren't going to happen. You build that trust by showing that yes, you know the consequences, you can explain and abstract them for everyone, player, artist, programmer, publisher, etc. And like, that's game design too! Your players and audience understanding what your game is, isn't some separate skill. Fundamentally, as a game designer, you are communicating ideas across so many different levels, from direct players senses, to language, to emotional and intellectual appeals. And systems are the structure you use to do that communication!
