I think the most important way to judge any art is by your reaction to it. What emotions does it bring out in you? Does it remind you of something? Does it represent its subject well? Is your interpretation different from the person next to you? The greatest pieces of art are the ones that force you to pause as you walk by them to get a second look. Personally, I also judge art based on the level of physical work that goes into it. The more work, the more likely I am to give it a chance. But that is a personal bias more than anything else.