I've seen a few reviews (online and from friends as well) that Divergent has lackluster worldbuilding, with not enough back story to explain why the world is the way it is. Now, I'm a total sucker for worldbuilding, so that had me worried, but I was intrigued enough by the concept that I plowed into it anyway.
That concept is that the society is divided into five factions based basically on personality - the brave and bold are Dauntless, the honest are Candor, the peaceful are Amity, the scholarly are Erudite, and the selfless are Abnegation. Every child chooses at the age of 16 whether they want to stay with the faction they were born into, or change into a new one, based on aptitude tests that supposedly show which one they naturally fall into. Our heroine Beatrice, born into Abnegation but uncomfortable there, turns out to be equally suited for multiple factions, making her Divergent, which is dangerous to the status quo. She keeps quite about her divergence and joins Dauntless; much of the book is taken up with the brutal training she and other initiates must go through to become full Dauntless members. Of course, things must come to a head, and it turns out that there's an insidious conspiracy by one faction to take control of the others and Tris is the one to stop it.
I actually didn't mind the worldbuilding that much. Sure, it doesn't go into the backstory a lot of how this society came to the conclusion that splitting up by personality type would be the best way to maintain peace and order, or what led to that kind of radical reordering of society, but we're coming at this from Tris's point of view, and unless it happened quite recently, she's unlikely as a teenager to have questioned very strongly how her society got to be the way it is. Not all of us are that interested in history, and she strikes me as someone much more interested in the present and future than the past. The book has an in media res quality that I actually kind of liked, and I believed the world as set out, even if it didn't have a lot of backstory.
I did have two niggles, though I ultimately enjoyed the book quite a bit as a quick, entertaining read. The first is that I found the budding relationship between Tris and Four agonizingly immature. I know they're only sixteen, and this is a YA book (I haven't read a lot of YA books, so perhaps this is just part and parcel of it), but every few pages, it's like Tris is noticing Four in a potentially romantic way for the first time. Again. And then they have a little moment, like brushing hands, where it's obvious they're attracted to each other, and then a chapter later, something similar happens, and it's like it's the first time again. It gets pretty ridiculous.
My other niggle is story-related, and thus pretty spoilery. The whole plot to take control is a bit too on the nose. I mean, the reason the faction idea is interesting is because on the surface, it does seem like a good idea - put people together who think similarly and you can get a lot done in that particular area, because you're always using people's skills to their best possible potential and in a way they inherently understand. On the other hand, encouraging people to think ONLY one way and crushing anyone who might be Divergent is clearly a way to enforce conformity and reduce critical thinking. That's pretty well stated in the book, that divergence is dangerous because it can't be controlled. But then the actual climax is far more mundane, with actual technologically-based mind control. No. The point of this is that if you develop a population with no critical thinking skills, you don't HAVE to use actual mind control - they're already predisposed to act according to the program laid out. That would've been a much more interesting way to go, I think, to explore the extent to which the faction system ON ITS OWN sets up a population ripe for exploitation by leaders who have the power to do it.
In short, the book is an entertaining read with a great premise, but it suffers from emotional immaturity and a desire to have a big climactic action set-piece rather than actually exploring the philosophical ramifications of that premise. That said, I'm pretty game to read the next in the series.