This is an amazing movie. I haven't read the books but even then I still got the concept and the reality. This movie isn't got everyone to understand because they weren't meant to. This movie is to show us of our actions now. I agree it should've been more lengthen. Or do I? See maybe the movie was shortened not just because of budget, but to show we our running out of time and of the harm we are doing to each other everyday. Some viewers think just cause they read the book that it had to be exactly like it. The director and the author of the giver are two different people. It's like in art, everything is different to every eye or viewer.
So as far as the the sequel is concerned I would love to see it as well as the last of the series. There could be more to just oh i didn't like it because it didn't have all the details or didn't go exactly like the book, but remember it takes days to weeks to read a book and also the movies are based on the directors view of the books and how he visualized it in his mind. And maybe my visualization would be different if read the book but I enjoyed the aspects and the story line.
Now I say this boldly, "Even if I were to read the book I wouldn't change my mind set of the movie or even what I said here!"