Sorry I don't have the time to detail everything out for you! You won't comprehend it anyway until you're actually out on the line doing IOE!! You asked what was so different from the sim... Nearly everything! In the sim they are preparing you to pass a check ride so it's nothing but maneuvers and approaches over and over mixed in with emergency abnormals. There is no time to teach you how to do actual line flying when it comes to dealing with ops, flight attendants, passengers, ATC, ramp, visual approaches, ect.... All of that comes from IOE where you will feel way behind the curve for most of your first trip... Then after you're finished you get dumped into the system where you'll fly with a myriad of different personalities and styles who aren't going to babysit you as much as your IOE captain does.... It will all start to slow down for you eventually after a while, then you can start to think of other things in the operation and not just flying the airplane... Ok?
Also I think what catches people off guard is the pace of everything, it seems like you're always rushed and so I think it can be easy to get frustrated. Some people catch on quicker than others..