I've posted some of these in another thread asking this, but I'm always happy to share again!
*the different walking sounds that your character makes based on if they are barefoot or wearing shoes
*how the aquariums in the museum get more decor in them the more fish you add to that particular tank
*seeing bugs hopping from flower to flower
*I noticed my villagers change clothes based on what activities they are doing and if it's raining or not
*all of the activities that the villagers do!
*the way the ocean turns colors depending on what time of day it is. I especially love the turquoise sea of midday and seeing the pretty sparkles on the water
A small detail the I have noticed is that when you talk to villagers either inside or out, the camera will focus on on your character and the individual animal! I makes me focus a little bit more on my villager and what they are telling me!
Using your reactions on villagers will make them react back
Villagers wearing accessories sometimes (well idk about the others, but I see Alice wearing glasses sometimes and it makes her look absolutely adorkable)
The 1am music is honestly so good, probably my favorite out of all the hourly songs. Makes me feel less guilty about playing at 1am lol
There's so many. Mairen listed out many of them that I've also noticed and love.
Though a new one I discovered today is that when two villagers have become friends with each other they'll greet each other when walking by. I honestly love small npc to npc interactions like that, it makes the game feel more alive.
Wind physics is crazy haha.... I love it! <3 Like clothes and flowers will flap. I got the elaborate kimono stand for the outside and the sleeves wave!!