Art exists in order to express ideas, and it does this through a specific means - means different from those used in other forms of expression.

This means that "good" art (painting, drawing, sculpture, literature, music, drama) is art which is effective at expressing its ideas.

Art can be subtle, complex, hard to understand, and difficult to explain and art - well "good" art - excites emotions. Whose emotions? Art critics? Experts? Historians? No, you the buyer, observer or collector.

While art can be emotional it is not incomprehensible. To say so is justification for "experts" to impose their opinions and judgments.

In practical terms knowing more about how a painting, novel, movie, or symphony is made (which may constitute a decrease in the amount of "mystery" surrounding it) never diminishes the ability to appreciate it. On the contrary, the more you know about a work you more you can appreciate it.

So why does some art elicit strong emotional response? Because it's good. (Just kidding!) Past the initial emotional tug, look at the work and remember that rational analysis and emotion should not come at the expense of each other.

I think you'll find that a "good" painting or drawing has a strong compositional design; intended colors and patterns; subject matter that catches and holds the attention of the viewer since one can't express anything to someone who is repelled or disinterested. The artist may use a host of techniques to advance its expressive purpose and may manipulate light or make the surface smooth, textured etc…

When the expression in a piece of art is weak, confusing, unclear, or prone to misinterpretation then it is not been effective. To me that is "bad" art.