If you're familiar with the book, I don't have to say much. It starts out the same way it does for all Gladers; not remembering anything, not even their name, briefly. All they remember of their life is being in that box with supplies and then seeing a ton of boys looking up at you and being called The Greenie until the next person comes. That's what it's like for Thomas, and the only thing that he can put that's different is that people seem to know him from somewhere; they don't exactly know where, plus Thomas is somehow so excited at the possibility of being a Runner. He's the only one who really doesn't care about the rules and puts himself over others if the time must come. He might be the first one to ever last a full night in the maze. Plus, pretty much every weird thing that starts to happen is related to Thomas in some way.
It's true: this movie takes some stuff out of the book. But that may not be the right description. I think they enhanced the Maze Runner source material's possibilities, giving even more tension and claustrophobia than you may have seen in the trailer. It finds a new way out, a new kind of key, a shorter length, a different story for Gally, and it all somehow comes together. It's even sometimes funny and intentional. It was so action-packed and mysterious that I wasn't bored for a nanosecond. I suppose the only thing that I didn't like going out of it was something the book did as well. It answered some of the questions I had, but by the end, it raised even more questions than how much I had when I walked in.
I guess I'll hopefully find out more in the sequel, and hopefully like it as much as this one, because simply put, book-to-movie adaptations don't get much better than this.