One of the things that players of games notice intuitively is their user interfaces. Small things make a game’s interface fluid or clunky, and ugly or beautiful. A designer who isn’t careful can introduce unnecessary elements or hide crucial information and functionality, crippling their game while working on a part of it that is often underrated. Furthermore, interfaces set the mood of the game before any of the other art and mechanics come to the forefront.
One of the easiest ways for a game designer to damage their games is to pay too little attention to the methods by which they design the mechanics of the game. The flaw that comes up the most when I see it is a failure to correctly scale games’ mechanical structures in relationship to each other. Often, developers get lazy and use exponential or even linear scaling in their games and don’t realize the impact that their actions have on the difficulty and balance of their work.