The way the novel presents society I found it really brilliant. You can read the novel today and be isntantly transported to a period of American society that is so different from the present day. I think that's where the "greatness" of the book lies.
On the other hand, all of the characters in this book are incredibly flawed. Even Gatsby, with his love for Daisy, feels like a flawed doomed character.However, I couldn't find a single character that I could actually root for. Maybe that's why some people don't find the novel to be so great.