I am very lucky, in that my first trip to WDW was in 1979, so there is not much I haven't been on. The few I missed that I wish I got to experience were:
-The train at Fort Wilderness. My parents were never interested in visiting FW, so it was the one place we did not visit during our trips. I always wished I had gotten to ride that FW train.
-Adventurer's Club. I actually was in here once, but I only stayed for around 15-20 minutes & then left to head to another club. What a mistake. I really wish I explored this amazing place & experienced the full show.
-Playhouse Disney: Bear in the Big Blue House. This attraction came about when I was on "WDW hiatus" & I missed it. Now I have a 3 year old who watches Bear episodes on Youtube & loves it. I wish I could take her to this attraction.
-Tour (or stay!!) in the YOAMD suite in Cinderella's Castle!! I know its still there, but oh how I wish I had won that prize during our YOAMD trip!!!
I think I have seen & done pretty much everything else that has come & gone in WDW???