It Was His Sled: The fact that Ember is an Underground City is not found out until the end of the first book, but good luck finding someone who treats that like a spoiler (Hell, it even says this on the main page!).
Paranoia Fuel: One doesn't have to be a rocket scientist to figure out that the situation in The Prophet of Yonwood (the U.S. battling a terrorist nation that threatens total war) is a natural progression from what the world is going through today.
The film adaptation:
Awesome Music: "One Last Message", perfectly fits over the credits and end sequences.
Doon swapping jobs with Lina, meaning she gets to be a Messenger like she always wanted. She sound so happy as she tells everyone what her new job is.
After making it to the surface, the children discover nothing but darkness and coldness. Then the sun comes up and Lina gets to see the sky for the first time in her life. The way she says "it's blue!" is so sweet.
Idiot Plot: There whole thing happens because the Buildersnote who are so talented that they could build an entire city underground and reasonably expect it to be sustainable for two centuries- thought it was a good idea to have the key to leaving the city sealed inside a box, and for it to be passed down from one mayor to the next despite none of them knowing what it was for. This admittedly is an Adaptation Induced Plothole — in the book, the box with the instructions was kept in the basement of the town hall, until it was stolen and taken home by the seventh mayor, who died before he could tell anyone about it.
Even negative reviews of the film had nothing but praise for Saoirse Ronan's performance. The first film she made after her acclaimed supporting role in Atonement, critics called it a sign she would have a promising career.
Harry Treadaway got plenty of praise too, and parlayed it into a respectable film career, and eventually a starring role in Penny Dreadful.
Moe: Little Poppy is just adorable, and thankfully proves quite useful when escaping Ember.
The sequence of Lizzie claiming she has a new boyfriend, only to reveal it as Looper. It's such an odd attempt at humor and it seems like such a Crack Pairing that it's bound to induce the wrong kind of laughing.
The filmmakers wanted to avoid modern slang, reasoning that Ember is several hundred years into the future. Unfortunately that results in a lot of characters speaking in lots of flowery big words like "dissuaded" that just sound forced.