While there are some small but significant liberties taken, IMHO, they actually serve to better relate the larger themes and emphasize the more obvious symbols and back story.
As mentioned above, the device of Carraway writing from an institution sounds very contrived in speaking or writing about it, yet on screen - it worked. Similarly, the flashbacks to Gatsby's childhood, Dan Cody and his days as a junior officer were brief but very effectively contributed to the overall telling of the story.
Makes sense to me. And FWIW (without having seen it yet) DiCaprio strikes me as the best possible choice to play Gatsby from the current crop of actors—maybe because of, rather than in spite of, all the reasons he is hated on FR. (I would have liked to see Allison Pill as Daisy after she did such a believable job as Zelda in Woody Allen's Midnight in Paris.)
I thought the flashbacks to Gatsby’s childhood were an improvement on the book. And in the book, when Carraway asks Gatsby what part of the Midwest he is from, Gatsby answers “San Francisco.” The movie omitted this ridiculous answer.