Ulgh. Falling into the 20k lagoon. I only vaguely remember seeing it in the early 2000s on my first trip, long after the subs were gone, but I'm deathly afraid of the idea of falling in. Even with it filled in, I still get nervous and I'd be even more nervous if it was still around. Being trapped in that water, or in that dark, abandoned cave with those frickin' scary squid. All my worst fears.
Anyway, as for lost favorite attractions, well, it's not too long ago, but I'd freak out if I could see the Pixar Block Party in person. I never hear many people talk about it, but oh my God, that "parade" was so cool. Just watching it on video makes me want to jump around and dance like I'm at a concert. Such an energetic show.
Of course, I'm very disappointed that I never saw the original JiI. I'd be bouncing in my seat and staring about in wonder if I could see the turntable and the science segments respectively. Also, special mention to Magic Journeys. I think it'd be a real, uh, experience to see that in 3D on the big screen.