I’m not sure how long or short my answers will end up being, so I apologize if I ramble.

This is a fun idea for a thread!
5 reasons why New Horizons is better than New Leaf, in no particular order:
1. Terraforming - Despite being a feature that I haven‘t utilized often, I think it‘s great! Seeing all of the different incredible builds and island layouts people have been able to create with it has been so inspiring, and I love the creative freedom that it gives us! I haven’t done much terraforming since it’s so far been too overwhelming for me between trying to decide what I want to do and actually doing the terraforming, but it’s definitely one of New Horizons’ highlights.
2. Villagers don’t move out without permission - Villagers not needing to ask to move out admittedly made cycling villagers a little easier in New Leaf, but it was needlessly stressful otherwise. I absolutely hated not being able to not play for a while without keeping note of the last day I played, unless I wanted to boot the game up weeks/months later to find one of my favourite villagers moved out. I’ve talked about this a few times, but not playing New Leaf for a few weeks due to a depressive episode and then finally booting the game up again to find Beau, my absolute favourite villager at the time, in boxes was heartbreaking. I’m so grateful that villagers can no longer move out without permission in New Horizons, especially since it’s common for me to go months without playing. It brings me peace knowing that even though I haven’t checked in on my villagers in ages, they’re all still there.
3. Going to Nook Mile islands to hunt for new villagers - Island hopping for new villagers is one of my favourite things to do in New Horizons! I think it’s so fun. I love the excitement of not knowing what villager will be on each island that you go to, or what kind of island you’ll end up at. My favourite is definitely shark island despite only being lucky enough to go there once, but I miss the hybrid island as well. I got a lot of my hybrid tulips from them.
4. Graphics - New Horizons’ graphics are so beautiful. They’ve always been one of my favourite things about the game. The sunsets and the Museum exhibits are especially gorgeous in New Horizons.
5. Being able to choose where buildings go/relocate them - Moving a new villager into my New Leaf town was always a pain in the ass because of having to plot reset to make sure their house didn’t destroy any items, paths or flowers, so I’m really thankful that we can choose where most buildings go in New Horizons, and relocate most of them later as well. It’s really made getting new villagers and decorating less stressful.
Runner-up reasons:
Wardell - One of the cutest NPCS ever, I love my quiet manatee!! I miss Wendell a lot, but Wardell is so lovely! I love he, I hope he’s in future games as well!
Character customization - I’m not the biggest fan of a lot of the hairstyles in New Horizons, but the character customization is still superior to New Leaf’s because you can actually
choose your skin tone. I think it’s ridiculous that it took until New Horizons for people to be able to have a dark-skinned character without having to tan. And the tans weren’t even permanent in past games iirc?
All of the new/returning villagers - Audie, Shino, Ione, Ace and Chabwick my beloveds
5 reasons why New Leaf is better than New Horizons, again in no particular order:
1. Tortimer’s Island, farming for rare fishes/beetles, and island tours - I miss Tortimer’s Island in New Horizons
so bad. I honestly could spend hours there in New Leaf, and I often did. It was one of my favourite features. I have so many fond memories of nights spent farming for rare beetles/fishes and filling my drop-off box. I also loved the island tours, even though I often did them by myself. They were still fun to do alone, and I think that speaks for how fun they were overall. I can live without Club Tortimer, especially since it sounds like it’s overrun by hackers now, but it was a lot of fun as well.
2. All of the NPCS that are missing in New Horizons - I miss all the NPCS that aren’t in New Horizons a lot, especially Pete, Pelly and Phyllis, Gracie, Phineas, and Dr. Shrunk. All of them helped give New Leaf the community-oriented feel that it had, and made it as great of a game as it was. Especially considering how much I adore axolotls and what my island name is, I really miss Dr. Shrunk. I miss learning reactions from him. I miss getting excited to see Phineas wandering around town and getting a new badge from him. I miss Gracie telling me how hideous my outfits are. New Horizons feels so empty without all of them tbh, despite how lovely the new NPCS are.
3. Pelly, Phyllis, Pete and the Post Office - Orville and Wilbur are cool, but they are no Pelly, Phyllis and Pete. I miss the contrast between Pelly and Phyllis; how sweet and lovely Pelly was, and how sarcastic and funny Phyllis was. I miss Pete delivering my mail. The postal system was also a lot better in New Leaf and prior games. You could store and send letters for free, and a 4-pack of stationary costed 80 bells. It costs 200 bells to send just
one letter in New Horizons. Writing letters also feels more tedious in New Horizons for some reason.
4. Online play/features were free - This might be more of a reason why the 3DS is better than the switch instead of why New Leaf is better than New Horizons, but I’m including it anyways. I miss the days where I didn’t have to pay for a subscription just to download custom designs, visit other players and have them visit me, etc. I can’t imagine Nintendo ever going back to free online play again, so the 3DS servers shutting down soon is especially sad.
5. Main Street - I loved Main Street in New Horizons. I loved having most of the shops and buildings in one area, and I especially loved getting to see villagers that used to live in my town there. It feels ironic having Main Street as one of my reasons considering I want more buildings on my New Horizons island, just so I have less space to have to try and decorate lol, but still. Main Street was great!
Runner-up reasons:
Town Hall/Train Station customization - The Town Hall and Train Station customization options in New Leaf were really cool. I wish we could remodel our Airport and Resident Services’ exteriors in New Horizons.
Small details - The small details in New Horizons are lovely, but there were a couple in New Leaf that I loved and miss. The campsite tent turning into an igloo during the winter months, and our character shivering in the winter months and sweating in the summer months when we first boot the game up and leave the house. Our character shivering after diving in winter was a super nice detail as well.
Perfect and tropical fruits - It’ll never not be crazy to me that tropical fruits were left out in New Horizons, considering that it takes place on a tropical island.
PWPS - I love that some of the PWPs came back in New Horizons in some way, but I’m bummed that some of my favourites were left out. Namely the illuminated arch and heart, and the tulip topiary. I also miss the fairytale bridge.
Playing hide-and-seek with villagers - The treasure hunt in New Horizons is fun, but I miss playing hide-and-seek with my villagers. I also miss when they’d ask you to bury a time capsule for them, and then dig it back up for them at a later time.
Working part-time at The Roost - I was really hoping that this feature would return in New Horizons as well when the 2.0 update was first teased. I also miss The Roost being a standalone building.
The small events - The smaller events like April Fool’s Day, Winter/Summer Solstice etc were a lot of fun. I really liked seeing the photo cutouts in the Town Plaza during a lot of them.
The town tree - The Town Tree was one of the most beautiful features in New Leaf imo. Watching it be planted as a sapling on your first day and watching it grow bigger and bigger over time was so lovely. I loved sitting in front of it and getting to look at the history of my town.
I’m probably missing reasons for both games, but that’s all I can think of right now!