I'll start with the things I did like, then go to the things I didn't like. I enjoyed some of the scenes, and some of the lines. I enjoyed the scenes that were -for the most part- the same as the book. I liked some of the cast too.
Now...unrelated to the differences from the book, some transitions seemed too abrupt, and the characters did not seemed very well developed. It felt like they got some of their layers peeled off in the movie.
The amount of content that was changed or removed that was in the book, really disappointed me. There were A LOT of things that were changed or removed. Some scenes that were so incredibly different from the book, were a bit painful for me to watch. Things that were removed were ranged from big to small.
*SPOILERS FOR DIVERGENT AND INSURGENT* One of the big things I found incredibly frustrating was the ending. If I remember correctly, Tobias shut down the simulation, not Jeanine. Tobias ran the simulation under simulation, not Jeanine. I don't even remember her in that scene. By keeping her in that scene, they could have just killed her and the conflict of killing Jeanine in Insurgent wouldn't even be there.
On the smaller scale, Tris didn't call Tobias by his real name after he revealed it to her. In the book, she calls him by his full name after she finds out.
Another thing that is a bit nit picky, but the smartest faction, pronounces their own faction's name wrong. Erudite is not supposed to sound like air-e-o-dite, it's supposed to sound like air-uh-dite. (I apologize if that was confusing, I tried to have the sounds as close as possible).
Thank you for reading, and I wish I could have liked this movie as much as other people. I'm happy for everyone who did and if anyone is thoroughly interested in watching it, go for it.