In Disneyland, other than the princess meet & greets, the only current attractions I've never experienced are Innoventions, the Main Street vehicles, and Storybook Land Canal Boats. I have no interest in Innoventions (other than to see it once before it is mercifully removed), and have simply not got around to the others.
In DCA, I have many of the same "misses" as others, including most of the kiddie rides in Flik's Fun Fair, the Golden Zephyr & the Jumpin' Jellyfish. I have not been on the Silly Symphonie Swings, but did ride the Orange Stinger.
Most of the "misses" on my list have only limited interest to me, but I still do not want to ever have to say I never rode any one that might be removed. There are several now-gone Disneyland attractions I never experienced although most, such as the Viewliner and the Flying Saucers, were before my time. But I do wish I'd taken a sail on the Mike Fink Keel Boats. However, I have no regrets about not taking a ride on the Superstar Limo. (Considering its short lifespan and even shorter lines during its active life, apparently nobody else misses it, either.)