I've been torn about this topic for quite some time, actually.
The first time I was at MK was in 1978 (I was almost 2 at the time). I went to Epcot the week it opened, and then I didn't get a chance to go back until 1984. After that, I went every other year or so until 1999, when I moved to Florida. I moved away in 2000, but I go back every year for the Food and Wine Festival.
Ok, so there's my background; true Disneyphiles should be able to gauge what was there when I was, because I can't really remember. I remember a skyway, but I don't remember riding it. Actually, I may only just "remember" it because I saw it in baby pictures. I remember riding Horizons right before it closed (I'm sure I must have ridden it before then, but I can't remember), but after viewing the tribute video, I can honestly say I only remember the HG Wells part in the beginning. I don't remember any of the rest of it, but until I saw the video I wondered where the black and white moon thing was from.
I don't remember World of Motion at all.
I have a Figment doll from the original ride, and I remember the original Innoventions (or whatever it was called then) from the same trip. I saw the tribute video for Journey Into Imagination and all that I "remembered" was the last room, with Figment on all the video screens. Oh, and the song.
I'm sure I must have seen Kitchen Kabaret, but I "grew up" on Food Rocks. I remember when The Living Seas opened, and I remember there was a ride, but I don't remember any part of it. In fact, I haven't stepped foot in there for the last 7 years, so I only have it on circumstantial evidence that the ride isn't still there
That said, I think the things I miss most are all the ones I don't remember. Everyone talks about the orange smell in Horizons - a tribute video doesn't give that. I wish I could remember seeing the Magic Kingdom from the Skyway. I really wish I could have seen Figment one more time before the Imagination Institute came along.
But mostly, I miss the feeling I had every time I went to Disney. As a child, I had no control over when I would be able to go. I *know* I must have drunk in everything on each visit, but I just can't *remember* it, no matter how much I try.
So, the most disappointing attraction closing for me was when I was walking along and hit by a speeding Mercedes and lost all of those memories.
(Don't feel too bad, I'm not disabled or in any other way visibly hurt; it just took away my childhood.)