Honestly what I miss most are the people who I love and had so much fun with that I went to WDW with all the time who are now deceased. I've been going to the parks since 1971, and honestly I don't mind any of the other changes. And if you go all the way back to them there have been a bunch. Although I will say lower crowds, lower prices, no need for any sort of dining reservations, and very few other people at rope drop were all pretty sweet.
If I get down to one picky little thing I miss, it would be the yummy coconut dogs they used to service at the quick service place by the Polynesian. It's been decades and I still miss those.