Thus far, the new stuff I've found out about in this game:
-You can cycle tools by tapping left or right on the dpad and put away any held tool by pressing down. This is a godsend timesaver.
-Villagers will now play hide and seek with you in an event that will interrupt gameplay and have them hiding around town. You'll get a prize for finding them all.
-Insects will now react to your net being out. Not only will scorps/tarantulas be harmless unless you have your net out, but some other insects will flee from you.
-Villagers will lose their housekey and you can fish it out of the river for a reward. Their door will be locked in the meantime.
-You'll need an "invite" to bring someone into Redd's little back alley shop, and after being invited, Redd gives you one as well which is actually a letter you can send to someone.
-The Resetti lair is reachable.
-Introduction of "Silver" tools as well, which have different effects from their golden counterparts.
-Today, a villager asked me what I was up to and my reply option was "That 'Thing'" to which he replied "Oh, yeah, that 'thing'". I have no idea what I just told him, but I can only assume it involves yiffing.
-Looks like you can talk to animals outside their houses before they move out of town (you won't always find them with their furniture packed).
I'll add more as I come across them.