I’m not a game designer, but 15 years after initially reading it, Jesse Schell’s “The Art of Game Design: A Book of Lenses” really sticks with me in any product design context. Organizing your thoughts around the lenses presented in this book makes for productive discussion, and can turn subjective points (an example might be how frictionless or not a UI element might be) into more objective points. I suggest checking it out. The author posted a deck of the lenses here: https://deck.artofgamedesign.com