Allegiant picks up with the characters' reactions to the amazing secret with which Insurgent ends...making it a little difficult for me to discuss here without spoilers. When members of the city think they've found a way to solve their problems, they soon find they have landed in a whole other mess. Let's just say, that a few more acts of revolution take place throughout the book.
I didn't read any actual reviews of Allegiant, only hearing people say "yay" or "nay" around the book world, but if I had to take a guess, I think what people may have seen in Allegiant is a winding down of action. What can you expect though? A book that is the end of a revolution has to wind it all down. I do think Allegiant is better than Mockingjay, the conclusion to the Hunger Games trilogy. I felt Mockingjay was rushed and wrapped up just for the sake of being done. Allegiant at least felt like it came together at a rightful pace for its purpose.
The other difference in Allegiant is the dual narrative. Tobias takes turns with the first person narration, instead of solely having Tris narrate as she did in the first two books. There is a very good reason for the switch to a dual narration, one I wonder if Roth didn't have planned from the beginning, which is why the dual narration only shows up in Allegiant. (Not to mention that there are four short stories written from Tobias's point of view coming this summer.) At first, it is hard to remember that someone else is narrating when one specific character narrated for two whole previous books, but not so bad overall.
So I liked Allegiant, although not as much as the first two. Oh, and I've never been one who needs a happy ending, but honestly, I was pulling for one here and well...you'll have to read to see if I got what I wished for!
Do you think the last book of a series is naturally different than the others in the series? Maybe it's more a trilogy thing?