The basic game hasn't changed, in the sense that it still feels like New Leaf. But yeah, there's a good bit of new things in the game.
- you can now hang shirts on walls
- you can sit on rocks
- you can unlock more storage
- you can stack fruit that is sitting inside of storage
- there's a MEOW coupon system. Basically the game gives you little daily objectives (catch 20 fish, build a snowman, sell stuff at Re-Tail, etc) and you get MEOW coupons for it. MEOW coupons can be used at the new campsite area (see next bullet)
- there's a new campsite area (different from the PWP) where characters can set up RVs. Some special characters will show up even without amiibos. Many RVs you can't get without amiibos though.
- there's more than 60 new villagers: 50 from the new welcome amiibo cards, 6 from the new Sanrio amiibo, and some from Zelda, Monster Hunter, and Splatoon amiibo figures. You can get the welcome amiibo villagers from someone else if they are in boxes/moving out in the other person's town (Sanrio ones are supposed to be scanned amiibo only)
- they added MANY new items to the game. The vast majority of them are non-reorderable though.
Most of the new items are only bought from the RVs with MEOW coupons
- some of the RV items are sold by Harvey (new NPC). You pay with MEOW coupons and generally it costs slightly more than buying from an RV
- there's a few new fortune cookie items
- They added a few new mini-games that you can play with the new Wii U and 3DS items. They are Island Escape (same as the one from Wii U amiibo Festival) and Puzzle League. Both let you unlock things.
- Wisp is now in the game with a magic lamp item. He lets you summon characters to your game with amiibo. Essentially, if you have amiibo, you can force any villager to move out and replace them with the character that is on the amiibo card. If you're an amiibo collector, this is HUGE.
There's more stuff too, but those are the more major ones.
I have 2 cartridges and decided to reset one of them so I could get into things easier that way.