I love so many things about New Horizons that limiting the list to ony 3 is very hard. I think, at the moment, this is my top 3:
1.- Various activities and differences of the islanders/animals/residents
My neighbors do so much. And what they do is as unique to them as possible. When I play, I can see my little animals not only do their preferred hobbies, but they can also be doing morning aerobics or yoga at the plaza, I can catch them dozing off at some hours, they are sometimes hungry and eating a little snack, I can find them reading something on their houses while having a face-journey...
They are so full of life. They don't just walk around, walk around holding a net/rod or water a flower. They have such a different range of activities and objects they carry around and that's what makes the island feel alive- they are not just favor-machines. Their personality and characteristics shine beyond their words, and these visual differences make them feel distinct from each other. Not all jocks are into fitness, not all crankies eat chocolate-covered donuts, not all peppies read comics, not all snooties eat chocolate popsicles during the summer, not all normals drink coffee.
10 residents of the island, each one doing something while I walk around is just so cool. I can sometimes just sit with my mom and watch Renée sing whatever K.K song is randomly selected or let her do yoga alongside Mathilda, Chadder or Melba. The time just slips as I watch Zucker and Angus struggle with their dumbbells while Antonio flexes at their side, watch how far Bianca and Boots zoom away or laugh because Julia is once again, inspecting the Gallant Statue with a magnifying glass.
In my humble opinion, one of the best things of the game that is also one of the most overlooked and constantly disregarded. I'd like to see this expanded on a future game even more, adding just 2 more snacks and drinks and 2 new hobbies would help to make almost all 400 animals unique.
2.- All the QoL improvements/features
One key reason I never bought NL was because I saw many limitations that although I would have eventually adapt to, I knew they would be frustrating. NH comes around and fixed almost all of them and here I am. Pocket space? Big, it lets me do a lot around my island without having to visit the shop/house every 10 minutes. Storage? Huge. I can be the little hoarder I've always been. I can run around like a madman without worrying about killing my flowers or ruining the grass. I don't waste time stacking everything, it is done automatically. I can now buy multilple things at Ables at the same time and I have a fitting room to try everything quickly. I'm able to choose my features without looking for guides online.
3.- Decorating the whole island
I spend 95% of my playtime outside my house. I don't care about my house as much as my island because of that. Of course I decorate my house and try to make it as great as possible... but the real fun is outside. I love watching an area I worked hard, I love running by the houses of my neighbors because I decorated them with items they would enjoy according to their tastes, dialogue, personality, hobbies... I love when the residents play the instruments while another comes closer to the microphone and starts singing. I love seeing them activate a loom, the monster statue, the drinking fountain... I love when I see them talking photos of the pumpkins, the little trees... And when they dance on the stages I have set? Oh my goodness. If I'm tired of the hourly music (which I'm not) I can also throw stereos around and play 90+ different tunes to drown out the music if I want.
Having the whole island be a representation of my creativity, of my imagination, of me is such a good feeling. And that's also helped by the fact that you're not limited to decorating with furniture items, you can do a lot with everything. I have spider crabs and turtles roaming the beaches, I have balloons swinging with the wind, I have a haunted floating statue just hanging around, I dropped a lollipop on my picnic area, a maple leaf umbrella is there being a pretty giant leaf on the path, I have two wedding tuxedos because heck yeah gay wedding on the beach. The placement of flora is just as fun... do I have enough cedar trees here? Maybe I should put another bush down the line. And the bonus of all? Everyone does it differently. There is so many ideas, so much creativity... the game lets you be so different and express yourself.
Extra bonus: I can immediately place an item I receive to see what it is, no matter where I am. The thrill.
I could also mention the tons of details and care the developers poured on the game, the music, the graphics, etc. But yeah, I think those three are my current faves.