Express pass is let in before standby for shows, so it actually takes up more time since to get any kind of advantage you have to arrive early. No reserved seats, just first dibs at seats. There aren't many seated shows though; I think just two year round, Animal Actors and Horror Makeup, plus the seasonal Grinchmas. I'm not sure that Horror has express. It's really quite useless for the animal show as it's a huge stadium with no bad views.
Must see queues depend on how passionate someone is about that IP. Basically, any indoor queue has things to see that are missed by using express. Cat in the Hat, Hogwarts Express, and Jimmy Fallon are exceptions. To get the full HP experience at Forbidden Journey, you really need to do standby once and express once. Express skips a few things but allows you to linger in each part of the line to fully experience it without constantly telling people to pass you. Hippogriff and Gringotts have no difference between express and standby. The other one with the biggest difference is MIB- without spoiling details, you skip the preshow that sets up the entire reason you're there, as well as a hallway with interesting things to see. The queues generally regarded as being the best themed besides HP are MiB, Mummy, Kong, and Spiderman.