All art creates an effect. Good art has one special effect in additition to all the other effects--they can create the sense in the viewer to be grateful toward the artist for having intended all the effects.

For example, I can make a painting that makes you want to turn away in disgust. But if I can make a painting that is disgusting but at the same time, you feel in awe of the painting's genius for being able to make you nauseous, then THAT would be considered good art.

This appears in a lot of dualities:

plain vs. subtle
distracting vs. intricate
dirty vs. sublime
conventional vs. classical