I could spend pages detailing the minutiae of how the book and the movie differ in this instance, all of the things that the movie messed up. Rather than nit-picking, though, I'd like to look at the bigger picture, how the message differs. And it all starts with the decision to make Jonas older.
At first, this seems like a relatively minor issue. Movies adjust details like this all the time. The developmental difference between 12 and 18 is significant, though. By making Jonas 12, Lowry accomplished a number of things. Firstly, it marked the community as "other". We're accustomed to facing a change in life, setting out on adult responsibilities, at 18. This is easy for us to understand and allows us to identify with Jonas in the movie. Seeing Jonas taking on such a level of responsibility at 12 is the first flag to the reader that something is wrong in this society. He is literally having to grow up too soon. Also, by making Jonas 12, his awakening to emotions and cultural memory is a clear parallel to puberty.
My other major issue is with the portrayal of the community. The book shows us a flawed but stable society. No, they don't have choice or colors or even real emotion, but they don't know any better and are entirely satisfied with their lives. They lack emotional pain or conflict. It's a peaceful place. Superficially, this is the case in the movie though we see a number of examples of discord, of governmental machinations. There is a sense of menace and foreboding in the community in the movie that simply isn't present in the book.
This all comes together to affect my final complaint: emotional impact. The book is very much about Jonas' inner journey, his slow awakening to all that is missing from their lives and his inability to stay once he knows the truth. It is his story so when he leaves, though there is danger of being discovered for a time, the final pages are still only about him. I'd go so far as to say that we only see Fiona and Asher in the early part of the novel to serve as a comparison to Jonas and the way he changes. The movie give Jonas a love interest with Fiona and interpersonal conflict with Asher. Obviously this is meant to make the plot more external and thus more cinematic but it does, then, weaken the original point of the novel. We see both Fiona and Asher choose loyalty to their friend over obedience, something they shouldn't be capable of per what we are told by the movie, as they get injections to suppress all emotions. Asher in particular should not be capable of that level of loyalty as he, unlike Fiona, has not skipped and injection. And it's not just these two. We see even Lily express a small rebellion in the final scenes. The final scenes of the book and the movie are vastly different, telling completely different stories.
If you haven't experienced either version of The Giver, you might be satisfied with the movie. If you, like me, enjoyed the book I wouldn't put it high on the list of adaptations to watch.