I loved the fact that Luhrmann decided to put actual quotes from the book into the movie. This seems to be a common stylistic choice of his (see Romeo and Juliet.) The novel is filled with beautiful lines, so it was nice to see the movie give a nod to Fitzgerald's writing. I thought the quotes were embedded very well, and they all made sense in the context of what was happening.
One change that I noticed right away was Jordan's character. Jordan appears far more in the book than in the movie-- she even has a relationship with Nick throughout the duration of the novel. This was probably taken out for the sake of time, which I do not entirely mind, but I liked their relationship in the book. Or rather, I liked the fact that they had a relationship. I felt that their relationship gave Nick a little more characterization in the book and made him seem like less of a background character. I still thought that the movie was successful without this relationship, though, because Nick was involved with so many other characters.
Another change that I thought was interesting was that, in the movie, Nick became an alcoholic after the events of his summer in New York. This really showed the impact that his summer and Gatsby's death had on his mental state, which I found brought a new light to his character. The only thing that didn't make sense about this, though, was that the movie kept the line where Nick says, "I've only been drunk twice in my life." I'm not sure if this is an error or just suggesting that Nick is not telling the truth, but it's not really a big issue.