So much has changed but I think at its core, much is the same.
I think for me personally, the change that feels the biggest is the 2pm park-hopping rule. I happen to dislike this limitation quite a bit, but it's workable. If you're a family who typically does park hoppers, you might want to look into that further to determine whether it's something you want to pay for or not.
No more Magical Express. Make sure to arrange alternative airport transportation. Personally I advise against Mears, as we had a really terrible experience with them when they stranded us at the resort upon our return to the airport.
Since you're going in December (btw, we will be there the same time as you!), you'll still need to make park reservations.
Genie+ is obviously a big difference, though I find it a lot easier in practice than it seems in theory. Familiarize yourself with it ahead of time but don't make yourself crazy worrying about the details. You'll get the hang of it when you actually DO it. Make sure you budget for it, especially considering the time of year your trip falls.
Be aware of the mobile ordering option. While I remember it being an option on our 2019 trip, it's EXTREMELY prevalent now, and very convenient. One random tip is to figure out which locations at your resort have mobile ordering as well and familiarize yourself with the menus, that way you can easily place an order when you're on the bus/monorail/skyliner back to your resort. It's nice to be able to just pick it up immediately and be on your way.
ADR's are now 60 days out, which I'm sure has already been mentioned.
Virtual queues are also a thing now. It's worth looking into how those work and figuring out whether it's something you want to use or whether you'd rather just go with ILL. Also know that once you get that boarding group you can use google to see what the average call time for your group will be.
Tons of new attractions and new places to eat, obviously.
Skyliner is new, which is mostly only relevant if you're at a skyline resort.
Splash Mountain is shut down for the refurb. Grand Floridian is under a pretty massive refurb which currently includes the lobby. Ongoing construction next to the Poly for the new DVC building is underway. I mention these in case they happen to impact any of your plans.