The jiggle physics applied to the handheld bags. They move around like an anime girl.
Having drinks like the Shamrock Soda or Sparkling Cider have its own animation to denote a toast, letting them be different from other drinks that are now drinkable.
Both the Cranny and Resident Services looking different depending on the season and event, sometimes it's even many small changes leading to the big event.
The way flowers and bushes shake when the beans hit them.
The sign outside of Able's creaking when it's windy.
I saw that when you exchanged feathers on NL, they didn't even bother to make specific models for each color, it's just a bunch of different-colored feathers. For NH, they took the time to make the appropiate models and that's such an insignificant but neat detail that showcases the love and care poured on this game.
Your neighbors mentioning how it's the first time they visit your home and also giving you their favorite music track when you visit them for the first time is very cute.