no autosaves and Resetti will chew you out if you reset/quit without saving (New Horizons added autosaves and almost entirely removed Resetti).
time travelling back in time relative to your last save will also trigger Resetti.
playable NES games. They can be hard to find, but you can play them if you get any (iirc you can also transfer the roms to your GBA/GBA SP to play on-the-go)
home customization is still a thing in the original; however you only have a max of 3 rooms (2nd floor, 1st floor, basement). However, you can’t customize your basement floor or walls and the basement won’t count towards your HHA score.
no “cloud” storage, all storage is in furniture items, which have a max capacity of 3 non-furniture items (clothes, tickets, flooring and walls are fine, but not furniture). This is what the basement is for. (It’s actually kinda fascinating how furniture storage works, but I won’t get into that unless you’re interested)
maximum of 15 villagers instead of 10.
you can ask villagers if they have any chores for you; this is actually a great option and I’m annoyed they removed it because it means you don’t have to repeatedly spam villager dialog with the hope they’ll eventually give you a task.
there’s an island you can visit with another house which you can use as storage if you want. However, it’s a bit difficult to access on emulators because you can only get to it if you have a GBA connected, so you have to either emulate a connected GBA or use gecko/AR code to permanently enable Kapp’n.
no town hall
there are cops (they don’t really do anything except tell you if there are any free lost items in the lost-and-found or if there are any upcoming events).
Tom Nook is the only real vendor, as afaik Mable and Able only sell custom designs. Additionally, instead of Redd, it’s Tom Nook who occasionally sells artwork for the museum.
There are probably others I’m forgetting, but that’s kinda the main gist of it.