I didn't have many expectations going into this, but that being said, I was still disappointed nevertheless.
I think one of the biggest weaknesses of this book was the lack of dimension. The characters fell flat, the setting fell flat, and the plot itself fell flat. I think the main reason why all of that occurred IMO is the world Roth created of the factions.
I understand Roth was trying to be creative with world-building, but each faction just ended up being stereotypical in my opinion. This ended up hurting character development, since the only characters who weren't like that were the Divergent. Even the villain - leader of the Erudite - was stereotypical. Not only were her headquarters in a library (which isn't that big of a deal - it's somewhat logical), but for someone so smart, she just happened to reveal her most important information on the phone as our protagonist was walking in. Not very smart.
I understand the reason Roth had the factions like this was most likely to emphasize how that kind of world was not working - how it was dystopian. It took away intensity that the book might have had though, if the factions had been less two-dimensional.
It would have also have helped if Roth had developed her world more. Take Erudite, for example. They're supposed to be the "Big Bad" in this book, and yet pretty much all we know about them is that they value knowledge, they're power-hungry, and they like modern conveniences. I don't blame them for that, but I don't fully believe how that it's enough motivation for them to enslave a group of people and kill another. I could
believe that they'd do it for modernity, but since it wasn't really explored, it just didn't hold enough value to me, and thus the actual climax itself felt anti-climatic. Yes, it's logical for them to have done it all for power, but none of it made it feel real or personal, or made me feel invested in its outcome.
This novel felt pretty character-driven from the start, but because the characters lacked dimension, the plot itself ended up lacking suspense and intensity as well. That being said, the plot wasn't very complicated either, and it was pretty easy to guess what would happen next.
There were also little things that didn't really seem believable. The idea of the factions themselves was a little out there, but not in a way that was chilling and still somehow believable. There were little things that characters said and did that felt a little out of place. For example, there's when Tris had to shoot Will. There was no build up to it. I realized after that if she hadn't shot him, he would have just crawled after her or would have done anything to shoot her, but that wasn't made clear, and it just seemed really anti-climatic. Tris's parents' deaths were also alright, but I didn't really care, to be honest. Part of it could be that with her father, he'd always just been the righteous guy from Abnegation who'd hated betrayal, and all of the sudden, he started to develop a little depth, but he was killed before it could be developed further. It just seemed like he was killed for the sake of being killed. The same goes with her mother, though she had developed a bit more depth that Tris's father had by then.
Another thing that also disappointed me was Peter. One of the reviews here talked about how Peter could have easily been another Divergent, and after reading it, I completely agree that would have been a more interesting plot twist. His not using the "tracking device" was a little bit of a twist, but he pretty much remained the stereotypical villain.
Overall, it was an okay book. Roth's particularly good at writing action scenes, and I definitely enjoyed them a lot, but they weren't enough to make up for the issues she had. My favorites were probably the scenes with playing Capture the Flag and at the Hancock tower. She also had an alright structure for the novel. It wasn't all over the place or anything, and while the crux of it didn't really begin until the very end, it was interesting enough, though it felt more like a prelude - which isn't necessarily a terrible thing.
I'll probably read the sequel eventually once it comes out, but I doubt I'll be in a hurry to read it.