A good espresso will depend on what coffee beans you use, but the final product should be judged on five qualities. There should be bitterness, but not too much, sourness in balance with the bitterness, a bit of sweetness, good body, and an aroma. In Italy espresso is usually made of a blend of dark roasted coffee beans in order to achieve these different qualities whereas in the US the trend of baristas is going more towards single origin beans highlighting the characteristics of that individual estate or place.