It is not necessary to be an artist, musician, filmmaker or author yourself to be an outstanding critic of creative work in these arts.
Critics are analysts and the best ones are expert in their area of critique. They are able to judge creative works within the proper context, and they are able to communicate their judgments to a variety of audiences in a useful and thoughtful way. Critics analyze creative work, and they serve as a skeptical but necessary audience. They perceive the layered nature of creative works where less informed people usually do not, and they can interpret the significance of creative work to a depth of detail others cannot. Also, they are (hopefully) better able than most to distance themselves emotionally from the work and from other factors that sway our judgment.
Highly creative people understand the context within which they create things, but they are driven to toss a javelin a bit further, not merely explain a message in a manner others can understand. This involves a completely different kind of activity and a completely different set of goals and priorities. The value and importance of artistic works can actually be unknown to their creators, and can be discovered long after the death of that person. (Think of artists who died in obscurity, only later to become eternally famous, or authors whose books inspire discussion and new works written years after they have died.)
There is of course a powerful and natural tension between creative people and critics, with the critics seen as a necessary evil. However, critics– even very poor and unjust ones– are essentially teachers, and skills in critique are some of the most valuable a person develops during his education, regardless of the scope or goal of the learning. Anything that anyone has offered as creative work can be understood better, appreciated more, improved upon, or find new relevance through critique, even (or perhaps especially) when people disagree or object to the critique.
In education, advertising is an excellent springboard for talking about the layered nature of creative works and how critique reveals these layers and gives us the vocabulary we need to respond intelligently to what we see, read or hear. I like print ads as examples because they’re familiar, easy to obtain and deceptively simple. Take just one of them apart yourself in a close reading of the image, and it’s easier to understand that shrugging off a critique as “just an opinion” or (worse) being won over by a critique without doing much thinking of your own are both exceptionally poor ways to judge creative works, or receive judgments of them.