In 15 years, a good amount has changed, but here are the biggest ones:
1. Magic Express is gone. You need to arrange your own transportation from the airport to your resort. Mears Connect and Sunshine Flyer are paid busses, there are also cabs, towncars, and Uber/Lyft.
2. Fastpasses are gone. Instead, you have
Genie+, a service where you pay by the day to book return times on your phone. There are dozens of
Youtube videos that explain how to use it. You have to buy it each day if you want it, book your first time at 7 am, then book your next one after using the first one or two hours after park opening, whichever comes first. Check out different strategies to get the most value.
3. Extra magic hours are different. Rather than different parks each day, all parks are open 30 minutes early for all Disney resort guests, plus on select nights a park will be open 2 hours late for deluxe and DVC resorts only.
4. The Skyliner is a new cable car with 3 different routes: Caribbean Beach to Hollywood Studios, Caribbean Beach to Pop Century and Art of Animation, and Caribbean Beach to Epcot with a stop at the new Riviera resort. What this all means is that it's now a lot faster to get from Pop Century, Art of Animation, Caribbean Beach, and Riviera to Epcot and Studios.
The biggest difference in your day to day strategy will be Genie+, so definitely learn the system and at least try it out one day, preferably Magic Kingdom day. Restaurant reservations are made 60 days out from your first day, and mobile ordering is available at almost every quick service restaurant and many table service restaurants for takeout, which cuts down on lines.
After hours events are a lot more common now. You get in to the parks later in the afternoon, then for 3 to 4 hours after closing you get to ride most attractions with almost no waits. These can be worth buying a shorter ticket and adding a party, especially if you really like doing attractions. Some of these are themed and have special parades and shows, like the Magic Kingdom Halloween and Christmas parties, and others are not, you just get short lines and an empty park.