One of the most unexpected things for me was the sudden appearance of sci-fi elements. They time-traveling and the aliens seem almost glaringly out of place in this book, but only at first. Once you get into the rhythm of the story and the message that it conveys, these elements make complete sense.
There are also a lot of different topics packed into Slaughterhouse Five, but they are only hinted at and not thrust into your face. Some of the topics even contradict each other, the main example being that there is an anti-war sentiment alongside a pro-war sentiment. I am actually glad that I never read and analyzed this for any class, because I think I would have ended up disliking it. This way I can reflect and re-read it at my own pace and pull things out of it without any pressure.