A Kung Fu form, like any other creative expression can be measured against a deceptively simple three-point standard.
First, it should be aesthetically pleasing. It is said that beauty is in the eye of the beholder but I think it is fair to say that beautiful works of creative expression possess certain core characteristics. They have balance: a mix of familiarity and surprise, stability and instability, even-ness and irregularity all of which conveys a sense of solidity with an element of movement. They have energy: expressed through the medium via color, contrast, texture, rhythm, melody, tone, pace (among others) to the five senses of taste, touch, sight, smell and sound. Most of us can only appreciate a narrow range of this energy; but people who are trained in a given art form or those whose senses are sharp (either naturally or through training) can appreciate a wider or even the full range of a creative work of art - so a trained musician can appreciate not only the sound of music but the taste, smell and texture as well. Lastly, creative expressions have depth: reflecting the range of energies expressed within and between the five senses as part of the whole. So, the depth of painting reflects the range senses the work touches (sight, touch, taste etc.), the range within each sense (i.e. the number of colors) and sometimes the range within a range (i.e. shades of blue, or light/shade).
In Kung Fu, the instrument is your body, mind and spirit, the medium is movement, and the canvas is the space around you. Each of the elements must balance and support the others to create a seamless mix. The result is a form and the aesthetic is flow. The form itself might be described as a river with some actions flowing slowly and seamlessly while others dance over shallow rocks or roar with explosive power. It is the mix of these elements within a balanced whole that gives beauty (and purpose) to the form.
Second, it should have a practical purpose. In other words, your form should be more than a well choreographed set of kicks, punches and rolls. It needs to do something. The what can vary - but in all cases where there is a real purpose - precision is the key. The most obvious "purpose" for most people is the form's (or an individual action's) potential in a fight (or self-defense scenario). This is an accurate, and valid point of view given the obvious visual evidence of the martial arts' application as a fighting (or self-defense) skill. If the form or action is not performed with precision - its purpose is weakened. Another purpose that is much less apparent, but no less important (perhaps even more important) is the internal physical impact of Kung Fu. The movements of a kung fu form provides a "internal massage" by moving bone, muscle and sinew - but they also activate and facilitate various physical, electrical and chemical processes that are key to keeping the internal ecosystem of the body in balance. Again, precision is the key for this purpose to be realized.
So...whether your focus is on the external aspects of fighting/self-defense or the internal aspects of the body, details matter. Every form, every action must be performed with your full attention and precision every time - if not you are literally just going through the motions.
Third, it should come from the heart (it should be real). When you practice your form, every ounce of your beings should be invested. Your form should be a physical, mental, and spiritual expression of your being. I once saw a movie about a young girl training to be a concert violinist. She practiced, and practiced, and practiced until her technique was flawless. When she performed for her teacher, he told her that it was the finest display of technical skill he had ever seen - but it had absolutely no heart. It was flat and emotionless. When you practice, when you perform, put everything you have into it and the result will be a work of art (errors included).
