I know it sounds like a non-answer, but there's really no way to tell you what is skippable. Everyone likes different things; what I might deem "Must see," y'all might hate to no end!

I myself have gone on rides/shows expecting to hate them/be scared half to death and wound up loving them (Test Track being one example). That being said, here are a few tips (more of a "what to avoid if you or a family member doesn't like something" guide than a list of attractions to skip):
If you hate going upside down, skip Rock n' Roller Coaster
If you hate speed, skip Test Track (65? mph), Everest possibly (50mph, I believe), and Rock n' Roller Coaster (60 mph?)
If you hate heights, skip Tower of Terror and possibly Splash Mountain; with Soarin', you could just ask for the lowest row (only 10 ft or so off the ground)
If you hate being in pitch blackness, skip Space Mountain, possibly Stitch's Great Escape, and probably a few others I'm forgetting
If you don't want to get wet, possibly skip Splash Mountain (I've heard varying things about how wet you get there), and definitely skip Kali River Rapids in Animal Kingdom
If you like history, make sure to check out Hall of Presidents in Magic Kingdom and The American Adventure in Epcot