@sainity This reply is especially for you since you're being so argumentative about it. Might regret getting into it but look: there is ABSOLUTELY a way to grade art objectively. How do you think teachers grade? Purely by bias or standards of realism? No.
I'm shocked no one in this thread has specifically talked about the elements and principles of art (or if you have and I missed it, I apologize). But let's get right to it.
The elements are as follows:
- color
- form
- line
- shape
- space
- texture
- value
These are the building blocks and what are considered the fundamentals to any piece. The more of these that you are proficient in, the more successful your art will be.
And now the principles:
- balance
- emphasis
- movement
- scale/proportion
- rhythm
- unity
- variety
The principles are how you apply the fundamentals. Again, the more you can implement effectively, the more objectively "good" your art is. Notice how it doesn't say anything about style--you don't grade on personal style. Any style can look good if you check off most of these boxes, whether it's realistic or not.
Art is comparable to a language and the point of a language is to communicate. The elements and principles are like grammar: you don't have to adhere strictly to it, but people will understand you a hell of a lot more if you do.
You shouldn't have to ask the artist. If the artist did their job, you would know. It would invoke those feelings in you just by looking at it. And maybe it's not the EXACT same feeling they're trying to convey, or maybe it doesn't resonate with you personally. But what makes a piece of art objectively good is whether or not it succeeded in communicating its message.
What forms of art are "better" than others isn't a useful debate to have, but that doesn't mean all art is created equal. Just because anything can be art does not automatically make it good. I hope this provided some concrete answers for you, since you seem to be struggling with the idea.