Cassia, Ky and Xander all act like totally different people without any explanation or reason. The characters I fell in love with in Matched are all jerks in this book.
Most of the new characters are superficially developed and only serve the purpose of moving the plot forward. The exception is Vick. Of course, he is suddenly killed off halfway through the book, just after touching my heart with his story of lost love. It reminds me of a Shakespearean scholar who once told me that Shakespeare had to kill off Mercutio because he was stealing the play from Romeo. There was no point to killing off Vick unless he was taking us away from Ky. It didn't change the symbolic meaning of the book, move the plot forward, or change any of the other characters. It was a pointless death and we already saw lots of other characters die pointless deaths at the beginning of the book.
The actual writing is not very good, especially when it was brilliant in Matched. In this book, Ally Condie doesn't take the time to show us what is happening. She doesn't give us enough details to lead us to our own conclusions about the characters and what things mean. She takes the shortcut of telling us how to interpret everything. It is annoying and comes off as condescending.
There are plot flaws in the book that Ally Condie uses when she wants and drops when they become inconvenient. In Matched Cassia learns her society uses colored pills to control people. When Cassia was very young, her grandfather told her she was strong enough to not need the pills. She reflects on that memory several times and avoids taking any pills. In Crossed, Cassia suddenly believes society's claim the blue pills will help her survive without food or water. She doesn't stop to consider the consequences or her grandfather's words before she takes a pill. Later, Cassia thinks about taking a red pill to forget the past twelve hours, but she remembers her grandfather's words and changes her mind.
There are also lots of coincidences that happen so often they become unbelievable. Cassia just happens to be placed in a camp near Ky's previous camp. The first words she says are "I am looking for someone. His name is Ky. He has dark hair and blue eyes. He came from a city, but he knows this land, too. He has words." Of course, one of the boys knows exactly who she means and leads her into the Carving. Later, half dead from poison, she stumbles up a cliff to see the sunset. Ky and Eli happen to see her and they happily reunite. These kinds of coincidences happen constantly. It would make a great soap opera because it is almost laugh out loud ridiculous.
Rather than take the time to flesh out the plot and the characters, Ally Condie tries to force the plot by having new characters just "figure things out" without any knowledge of what went on before or using the clues we have. Indie suddenly knows everything about Ky and Xander, even though she just met Ky and has never met Xander. Since I didn't have any of the clues Indie used to "figure it out", I felt cheated, rather than having an AHA moment of "Oh, that's what that meant." I felt like I was ten again and hearing Darth Vadar say, "Luke, I am your father."
When Cassia finally finds what she is looking for, she is sent back to society. Coincidentally, society hasn't even changed her status, so in the third book she can be back with Xander like she never left. She risks everything to find the rebellion and then happily lets them sort her back into society without a word. So much for all the times she and Ky swore not to go gentle.