I started thinking about this question after reading Giuseppe Tomasi di Lampedusa's The Leopard, which I would have to say is worthy of the title of a 'great novel'. Words like 'classic' and 'great' are overused, and the only antidote is to think about why we're applying them. But the answer can't be simple; it obviously has to be a combination of things. One is definitely the ability to stimulate your visual imagination. Tolstoy did that for me; from the first page of War and Peace it was as if I could see the scene in front of me, the description was so vivid. And Lampedusa does it with his descriptions of Sicily; of the landscape, of the buildings, of the weather, and of course of the people.
Another has to be characterisation; the characters have to be absolutely themselves, yet they also have to stand for something. If all they are is figureheads, then it doesn't work. It was said to me once that fiction allows the potential of reality to be explored without the interference of contingency, and one of the things that this seems to imply is that there is a logic of events that is involved; in the case of The Leopard, the move from a world dominated by the aristocracy to one in which the bourgeoisie holds sway. Because there is no commitment to evidence as there is in writing history, the full implications of these contrasting principles of social organisation can emerge clearly.
Yet at the same time it seems to me that a great novel also has to avoid over-simplification, and The Leopard does this. Though written by someone who by birth could be expected to sympathise with the end of the aristocratic era, it no way pretends that the world that is vanishing is superior in all respects to what is taking its place. One way of expressing this might be to say that there is a difference between the narrative voice and the authorial voice; that, indeed, the authorial voice, insofar as it is merely personal, recedes entirely, so that one is never really sure (one perhaps does not even need to care) what the author himself actually thinks - the narration is never really exclusively identifiable with a particular point of view, or exclusively sympathetic with (or condemnatory of) a particular character. So although in a way a great novel clarifies reality, it does not over-simplify it so much as it disambiguates it.
Then there is the ability to convince the reader that this is what it was actually like to be alive at that time. This is of course entirely groundless; you can never really verify the accuracy of a representation of subjective impressions, because they are precisely what don't survive. But it isn't history, so you don't have to. You just have to create a convincing illusion. And this goes along with creating the illusion of a world that is internally coherent so far as it goes; everything seems to fit, to belong. Moreover, this world should have a variety of layers to it; The Leopard is a political level at one level, but at other levels it is do with manners, with values, with love, with death, with ways of life, with, indeed, 'world-views'. These at any rate seem to me to be some of the things that make it worthy to be called great.
