Perspective, depth, and angles. Like a book you must make your reader believe they are 'experiencing' your art. Even stick figure can be magical if they are done right. Many people get the idea that art can only be done one way, that it, like every thing else, is set in stone. That is SO wrong! Art is an expression of ones self, and therefor NEVER done the same way. The power and creativity of the right side of our brains is vastly underused in todays society. So no work of art is truly greater than another, but there are certain art forms that are more pleasing to the eye and therefor given the upper-hand and the museum walls. The only way to master THESE is practice.
I think that great art either causes a viewer to think or to feel. If it doesn't stir something up they may say 'That's nice' and move on, and wouldn't walk 10 steps to look at it again. In my opinion great art can be any style or technique or level of skill, but to qualify as great it has to create a substantial amount of activity in the viewer's mind or heart. Good art can be a matter of good concept or excellent skills in execution, but I think great art touches the mind, heart or soul of the viewer