Obviously a first criteria for a great book must be that they are written well. Language is a powerful weapon and it can be wielded expertly to weave a great tale. However, there are a plethora of great writers in the world but not all of them write great books. Certainly all great books must also have great characters. Not necessarily likeable characters, but characters who you can care about. Characters who evoke enough curiosity to keep you reading. Kafka's the Trial and Dostoyevsky's Crime and Punishment had nearly loathsome main characters, but they were vague and vile enough to keep one engaged in the book. Voltaire's Candide had flamboyant and silly characters that keeps one entertained as they read on. On the topic of Candide, it can be considered a great book because it's plot line kept the readers attention. It had a plot that had twists and turns and the unexpectedness of it all engaged the reader and beckoned them onwards.
Plot, characters, and an all around well written book are all things that no doubt can be credited to a good book, even a great book. To truly have a great book it needs to transcend time. To be tested by the readers of it's present and the readers of it's future and still be held up as a truly great book. Many great books were those that dared to try something different, and of course, succeeded in it. For example, Kafka's The Trial is by no means a normal book. It's setting is surreal, and it's way of the world seems unrealistic, but it's laden down with social and moral commentary. The chapters were left unfinished and are oddly arranged. If being a great book is succeeding doing something differently, then Kafka wrote the greatest books of all.
Great books can also be classified on the level of discussion they arouse. Good books can call forth a rather simple discussion of the plot or the characters, but great book challenge metaphysical boundaries. Often great books are filled with thoughtful content on conceptual ideas or deeply riddled symbolism that may just be the readers thinking too deeply into it. These are great topics for discussion, ones that can go on forever and spread into other examinations of the human experience.