A fascinating question. I suspect that art appreciation might well be important, although perhaps only as a symptom of an underlying difference.

Let's look at the question more generally. It is important for us to know what are the essential differences between humans and other animals for two reasons. First, because it is an important part of understanding who we are. Second, because we eat animals, wear their skins, keep them in zoos, experiment on them and so forth -- all things that we tend to feel are morally wrong with respect to human beings.

Philosophers, then, tend to be divided into three very general camps. 1. Those who believe that there are morally significant differences between human beings and animals. 2. Those who believe that there are not such differences, and thus tend to argue for animal rights. 3. Those who feel this is the wrong question to be asking. Here, we'll ignore the third group, for simplicity.

The most common distinctions given by philosophers in the first group are: reasoning, especially abstract reasoning; language use; moral reasoning and action. The 18th Century philosophy Kant argued that the third of these was the most important, because only this difference could itself be a morally relevant difference. We have obligations to humans, partly because humans are capable of having obligations. The fact that a cow (say) cannot speak would have to be supplemented by an argument that this lack is morally significant, and I am permitted to eat it.

Art appreciation, as a capacity, is much less commonly proposed. It is also very difficult to determine whether a cat, for example, appreciates or even makes art. The capacity for abstract thought seems much easier to decide.

One final point: you say that the difference between low and high intelligence is irrelevant. However, significant differences in intelligence may not be simply a question of quantity, but rather of type. Let us say, for argument’s sake, that a human brain is structured pretty much the same as a cat’s, but has 20 times the number of neurons. Now, does this mean the human is 20 times as intelligent, or rather that the increase in neurons yields a different kind of intelligence, which is not on the same scale? The smartest cat ever could not take an IQ test; not because it is not smart enough quantitatively, but because its intelligence is of a different qualitative type. This different type of intelligence might also be what is manifesting itself in the capacity to make and appreciate art. However, even if we accept this, again another argument would be required to show that it was morally significant.