There are a few things I still haven't done in WDW. I've actually never ridden Space Mountain. Have been saying for a handful of trips now I would go on it, but something always happens. Run out of time, I hurt my back, it was being refurbed, etc. On a non-ride/show note, I have wanted to play mini-golf at Fantasia Gardens, take a boat out on the lagoon from Wilderness Lodge, and take the Behind the Seeds tour at Epcot for YEARS, and it never happens. Either a bad weather system hits, one of us gets sick, or some other bizarre thing occurs to keep us from doing any of it. I'd say maybe next time, but at this point, I think I'm ready to throw in the proverbial towel...
In the last 2-3 years, I finally rode Universe of Energy, Spaceship Earth, saw the new Canada CircleVision 360 film, and saw Flights of Wonder in AK. They were things that always got put off for other rides/shows. I also finally rode Haunted Mansion this year after refusing to go on it before now; enjoyed it more than I thought I would.
Only other rides/shows I haven't seen at WDW (not counting holiday events) are Rock 'n Roller Coaster, Mission: Space, HISTA (if they ever bring it back), and Dinosaur. None of those are my thing, so I'll never be marking them off the list.