There are three main issues with this book, the first of which is a MAJOR SPOILER, so please don't read further unless you are okay with MAJOR SPOILERS.
Tris, the main character, the person you are reading the books to become, the heroine the books makes you fall in love with . . . dies. She dies.
Which leads to the second problem. She dies for a stupid reason. Now I know that Ms. Roth is a known Christian and there is a buzz out there that she sacrifices herself to save the world and her brother and all that in a very Jesus-like way. Or even in a Harry Potter-like way (except JK Rowling understood the rules of her genre and made sure Harry actually survived, which was kind of key.)
The thing is all of this is off base. Tris dies to advance a plan that seems pretty far-fetched to me, lazy almost in terms of plot and certainly a plan that could have been reached other ways if the author wasn't in such a hurry to kill off her main character.
And even if we pretend for a minute that the sacrifice was necessary to meet the goal, it wasn't saving the entire world. It was like altering a slice of the world. Kind of.
Which leads to the third problem. The plot makes no sense and the pacing is terrible. The introduction of layers of explanation for the outside world, the new lies on top of old twists and the supposed orientation of the entire universe just ends up feeling corny and wrong. For most of the book they sit around and snarl about this, go off on silly chases and then for nonsensical reasons two important people die. Then it wraps up with a bunch of whining from Tobias.
No. This is not how I wanted this series to end. In fact, I'm going to go to sleep right now and dream a different ending. Anything I come up with will be better than this.