How often does anyone really like the movie better than the book? We see the preview to a movie that we have read as book, a book that we loved greatly, and end up getting our expectations too high. In the end we go in wanting it to be perfect, and that's not possible. Overall, it just isn't likely to make a movie as good as the book without creating dull or unlively scenes.
The Great Gatsby was a well known and extremely popular book. That is a lot for a movie to live up to, especially with such a wide fan base. The movie has potential to be done very well, but I do not believe people ever really like the movies better than the books.
Movies (although entertaining) tend to leave out great detailed description that the books do not. It is kind of a let down when you read a book and have this idea of how the scene in the movie should go down and then it does not go down that way. They say there is nothing like the original and this is usually the case when it comes to adapting books into movies.
In the case of the Great Gatsby the book is filled with unique and personal imagery that each individual builds upon and pictures in the theater of their own mind. Whenever you ask a director to put his spin on a movie, the story suffers for doing so. Not to say that many of the visual elements can not be shared. The other factor is that a book is read over days while a movie has to fit all the information in a 2 hour block. The element of narrative and story suffers due to the time constraints. Will it be a bad movie, probably not, but will it be as good as the book definitely not.
When you compare a book to a movie, there isn't a basis for comparison. A movie will never capture the exact feel of the book. Forget about scenes that are cut or added. Even if you film every scene in the book, you cannot get the emotional impact of every part of the story.