The biggest thing for me is furniture sets. What I was expecting coming in to this game was that there would be one big "pool" (for lack of a better word) of craftable furniture, and another equally big "pool" of non-craftable furniture that you could buy from vendors, whether Gracie, the Nook Kids, RVs, Harvey's, island gift shop, etc. This obviously didn't turn out to be the case, and ended up being one of the deciding factors for my returning the game.
I'm just baffled as to why they cut so much. Even if they wanted most everything to be craftable, there were plenty of traditional sets that could have been made craftable (Alpine, Cabin, Ranch, Minimalist, to name a few...) that were just unceremoniously dropped. I've heard a lot on these forums that the furniture sets had to be changed to reflect the game's outdoor-oriented design philosophy, but even then, they were able to adapt certain sets to the New Horizons style (like the various holiday sets). Why not adapt all of the past sets and add more, rather than just replacing most of the old stuff with new stuff? More options are always better, for both indoors and outdoors! More items in general leads to a deeper breadth and variety of choices for designing, which leads to more diverse and interesting islands in general. When every other game up until now (as far as I know) has included older sets in addition to adding new ones, why is it suddenly too much to ask now, on the most advanced Nintendo console yet?
One thing that I'm the most confused about is the seeming arbitrary-ness (I'm pretty sure that's not a word, but oh well) about the choices of stuff that was cut vs kept. I'd like to know why some things were cut altogether, other things were sort of kept, but with a different name (Cabana becoming Rattan, Exotic becoming Imperial, Lovely being inverted and renamed Cute), and others got the seemingly arbitrary Seal of Approval to come back, some practically untouched (Mermaid and Horoscope are the first to come to mind, there may very well be others I'm forgetting).
I would love it if some day they released a big update, Welcome Amiibo style, that included a bunch of furniture options (not all just sets, mind you - I know I've been focusing on the sets, but the different sorts of general, unthemed furniture released in Welcome Amiibo that you could mix and match were awesome and would be great to have again) that you can buy from vendors. If they're too worried about crowding the island with shops and making it seem too "unnatural", then there could be some sort of "shopping center island" with Gracie, Amiibo vendors, and/or other NPCs. Kapp'n could even take you there, just to kill two birds with one stone!
In spite of all my ranting, I don't think this is likely to happen and is mostly just wishful thinking. Maybe they'll bring back the Gracie sets, but at this point, I would not bet on everything that was cut coming back. If it did happen, and some day I could have my Sweets, Gorgeous, Princess, Rococo, Regal, and Stripe sets back, then I'd be more inclined to pick the game back up and give it another try. Also... RIP my beloved, gorgeous cicada stereo.
The second thing I would like to come back is the Tortimer Island. I like that you can find villagers to invite to your island through the mystery islands. However, I was disappointed by the variety of mystery islands. Most of the time, I'd spend my nook miles on a ticket, only to end up on an island with the same fruit/flowers/bugs/fish as my own island, which became disappointing quickly. The excitement of knowing that you could land on something really cool and unique (like tarantula island, which did happen to me once) is nice, at least. But I wish there was an option, in addition to the mystery islands, to go to one Tortimer Island-style island that's always the same layout, including a gift shop and Summer fish and bugs to catch, just like in New Leaf. Maybe make it more expensive than buying a ticket to the mystery islands, but I missed the reliability of always knowing what was going to be on the island at any time, and the relaxation of fishing and bug-hunting late at night on the island for sharks and beetles.
Thank you for reading all of that, if you made it. At the end of the day, I suppose I just have to accept that New Horizons is just not for me as it is. It's hard to move on and take the new game and its own furniture for what it is when all I can think of is how gorgeous some of my old favorite items would have looked in HD, New Horizons style. I doubt they'll add all of my favorite furniture sets and individual pieces; they've made their decision and seem to be sticking with it, with a few exceptions. It is what it is, even if it makes me a little pessimistic for the future. For now, I'll just keep on playing New Leaf and lurking around the New Horizons section to see what all is getting added. At any rate, thanks for making the topic and for all those participating!