Before vs Now
A few 3d models here and there , a contrast color theme and a low poly feel can bring out a game alive!
Making a visually attractive game is much easier than what we might think. You don't really need to hire a 3d modeller or a concept artist. A casual game can easily go away with assets that are available on various stores at reasonable rates.
The same piece of code can feel totally new and awesome with enhanced graphics. So go ahead, don't shy away from trying your hand at visually designing your game. There's always a creative side within you that urges to try itself. Let it flow :)