Can You Say Cliffhanger?
After Tris ruins Jeanine's plan to use the Dauntless against the Abnegation (and has evidence of that plan), Tris and friends jump on a train heading out towards the Amity compound. And we're using the term "friends" loosely here, since the group consists of Tris, her brother Caleb, Four, his abusive dad Marcus, and jerky Peter. Tris's parents sacrificed themselves to save her and she shot a mind-controlled Will to save herself. So, things aren't so great, and her crew is rather motley.
On the bright side, Tris and Four are together and confess their love to each other. Does that make up for losing her parents and killing her friend? Hardly. But we'll have to wait for the sequel to find out how Tris emotionally handles the whirlwind she's just experienced.
On top of that, we have to read the sequel to find out what happens with the factions. Because at the end of this book, Tris gives us one paragraph that reminds us that "Abnegation and Dauntless are both broken, their members scattered" (39.87). Abnegation's leaders are dead or scattered; the Dauntless soldiers seem to be a little upset and confused about the whole "being mind-controlled into killing some people" and we don't know where their leaders are, nor do we know how those dynamics will shake out.
If there's one thing we learn from the novel's ending, it's that things can no longer go back to the way they were. With Tris's help, this world has reached a point of no return, and is either headed for total disaster, or total redemption.