It's a little different for me now. Since I was there for 5 months, visiting at least one park almost every day, I've done almost everything. Most things I've done alot. Plus some lengthy visits since then, I can miss almost anything and be okay, it's a different scale for me. Like my last trip. I was there for a week and a half and didn't do Thunder Mountain even once. So there are rides I have to do, multiple times if possible, those I have to try and do at least once, those that are worth doing if I have time, those I'll live if I miss, and those I avoid like the plague. So, trying to think as someone who has a little less time to spend there, I'd be fine if I didn't do:
Kingdom
Tiki Room (avoid)
Magic Carpets of Aladdin
Hall of Presidents
Snow White
Peter Pan
Small World
Winnie the Pooh
Dumbo (avoid)
Barnstormer (avoid)
Stitch's Great Escape (avoid)
Carousel of Progress
Astro Orbiters
Indy Motor Speedway
Epcot
Ellen's Energy Adventure (avoid)
Mission: Space
Honey I Shrunk the Audience
Living with the Land
Any show in World Showcase
Maelstrom
MGM
Anything that's not Muppets, Star Tours, Great Movie Ride, Rockin' Rollercoaster or Tower of Terror.
DAK
Pocahantas and her Animal Friends (still haven't done it)
Triceritops Spin
Kali River Rapids