The answer to this question is going to depend on each reader. It is entirely up to you to pick a reason for why you did not like The Giver. Maybe you liked the story as a whole, but you dislike a certain part. That's fine too. Just be sure to explain why you didn't like that part.
Personally, I don't like the book as a whole, because it is about a dystopian society. This is just my personal opinion. I don't like reading books or watching movies that show the future in a depressing light, because it's depressing. It makes me think that no matter what the actions of today are, they are pointless. Society is going to fall apart. I'm not so naive to think that the future will be a perfect utopia, but stories like The Giver, The Maze Runner, The Hunger Games, Mad Max, and Waterworld don't leave a lot of room for hope.
Jonas's society doesn't appear to be a dystopia at first. There is no hate, poverty, starvation, or unemployment. Those things are all wonderful; however, the people had to give up freedom and individuality. They even gave up changes in weather and the ability to see color.
I also didn't like the end of the book. The reader is left wondering what happens to Jonas and Gabriel. We never know if they made it or not. I can't stand endings like that. I just want the author to cleanly wrap up the story. The Giver does not do this.