(1) I'm with you OP - I miss River Country. Logistically, it would be a nightmare now because when I went as a kid, you were staying at either the Contemp, Poly, GF or the campgrounds, so getting there was no big deal. I have no idea how they'd bus everyone in now?? But, for what it's worth, I loved it and am sad every time I look at the area now.
(2) Ditto on figment. I have so many wonderful memories from that ride and the upstairs part. I remember being so disappointed when I took my DD the first time after I hadn't been to WDW in about 6-7 years, and it was just not at all what I remembered or had hyped it up to be to her and my DH (who had never been to WDW before).
(3) I miss the simplicity. No, WDW has never been a relaxing trip, but it's gotten overly complicated with all the reserving. I don't like to be scheduled on a vacation. But, the reality is if you don't get on board and reserve everything, it's your loss in the end.
(4) I miss Frontierland. Yes, it's still there technically, but it's dead. The Golden Horseshoe show is gone, the old time gun battles in the streets aren't done anymore. Probably because one crazy PC parent fought that cowboys and gun fights were bad for kids.
(5) I miss all the unique experiences, like waking up Tinkerbell or finding the paint brushes on Tom Sawyer island. I loved that if you cared enough, you could learn little things about WDW that made your trip that much more special. Seems like they cut all of that out too. Now everyone just gets the same stickers over and over again.