Great art is the product of any combination of factors. There is no one thing, or one attribute that will determine if a piece is great. Before getting into what can make a piece of art great, and examples of great art, let's think about the question itself. What does great mean? Clearly it is a subjective assessment of something of someone. Ultimately, it is a collective cultural assessment. If enough people agree that a work of art is great, then it is considered that, but an individual can dispute that judgment.
Take for an example Andy Warhol's work, notably the Campbell Soup Can. I myself have never been a fan of Warhol or his work and I am certainly not going to call him great. That being said, there are those who do. Why? Because he was an innovator. He did something that no one did before and in so doing created the movement of pop art. He had the idea of elevating the every day, the common, and the popular to artistic levels. He looked at what was art, though and created something different. That is why people respond to him as they did. As an old art teacher of mine once told me, "Anyone can draw and paint, but it takes and artist to be creative." Like it or not, that is what Warhol did, he created something no one else did before him and thus greatness had been inferred upon him.
Creativity is certainly a major attribute of great art. Art that changes the game, particularly in the 20th century, can be considered great. Duchamp's urinal, Picasso's cubist paintings, Nevelson's modular sculptures can all be considered great works because hey changed the perception of what art can be.
Innovation of ideas can lead to greatness, but so can innovation in presentation. Take for example Filippo Brunelleschi's innovation of 1 point perspective. That innovation is one of the defining moments of the renaissance and renaissance art, leading to such great art as Ghiberti's second set of baptistery doors. Speaking of renaissance art and presentation Michelangelo's work is of course considered great. Every sculpture he created is dynamic and full of life. He was so technically proficient that many contemporaries look amateurish in comparison. I bring this up to demonstrate another point of greatness, presentation.
To me these 2 aspects are the yin and yang of the equation, to create great art you need a mix of both. It doesn't necessarily have to be 50 50 but a combination is required. The greatest idea in the world isn't worth anything if it looks bad or is portrayed poorly. On the same note the most technically proficient painting or sculpture can be boring if there isn't something interesting behind it. When the right combination is struck, great art is created.