I found Divergent to be a very quick, very hungry read. I kept promising myself "one more chapter" until I had devoured the book within the space of one day. Sleep was had, but mainly because I started reading the book at 1am, read until I could no longer keep my eyes open at 5am, and resumed reading when I woke up some time in the afternoon. 
This doesn't mean it was without flaws. There were rare occasions of editing missing tense changes, but they did happen. 
One of the things that stuck out most about the wording of this book was the use of "breaths". Every time the author spoke about someone's breathing, she always used the plural form of "breaths". Now, it could just be that these instances stood out to me like a sore thumb, but I couldn't help but wonder why she might not mix it up a bit. "Breathing" or "Breath" would have fit some instances far better, and would have made the sentences a little less stilted and repeaty. 
As to the plot: I did find it a little hard to see how the logical division for society was into five groups, each group believing that war was the result of one particular thing. 
Ok, no, I lie. I didn't find that as hard to fathom as I did the idea that people are supposedly so very clearly one faction. And that someone who isn't as clear is "dangerous". I believe that certain things in the book further added to my confusion on this matter. Why, for example, would you test kids,  tell them where they should be, and then let them choose? 
Not from the faction they were born into and the faction their tests suggested they should go to, but from any of the five factions? 
Surely if a person is from Abnegation and their test suggests they belong in Abnegation, their choosing of another faction would be seen as out of character, weird, and also be seen to weaken the factions, which so pride themselves on a certain characteristic? 
If you're born Dauntless, your test tells you that you seek knowledge and should be an Erudite, and you choose to join Abnegation, isn't that just a recipe for disaster, for when you get bored, later down the track? 
I understand that the initiation is supposed to weed those out who aren't cut out for that faction, but the tests are different for different factions, and I... I can just imagine a certain amount of unrest once the honeymoon period wears off. 
I guess I find it hard to see people as JUST selfless, JUST brave, JUST kind, JUST smart, or JUST honest. I know, upon finishing the book, why a certain someone hates Divergents, but I just don't understand how it has supposedly been such a dangerous thing for so many years. Why EVERYONE sees them as dangerous. 
Basically, so long as you don't logic too hard at the book, it's a great, fun, fast paced read, though I do wonder if the lack of shock at certain deaths and other disasters is because there is an awful lot of it around, or because of the style of writing. 
I mean, there was an awful lot of death in The Hunger Games, and that had me bawling my eyes out in certain places. I didn't shed a tear throughout Divergent, though I did sometimes find myself worried for the characters, or annoyed at the heartless things they did. 
In the end, I read it in a day and I didn't want to put the thing down, so now you just know I have to read the next two.