Since 2012, I believe a LOT has changed, but my years might be mixed up.
DAK
-Camp Minnie Mickey is closed, check to where those characters have been relocated throughout the park, Mickey& Minnie are together and a fast pass gets them too
-Festival of the Lion King has moved back over to the Africa region
-No more "poaching" plot line on the safari
-Harambe Marketplace is open, an open aired market type place for food & drinks, the watermelon lemonade here is DELICIOUS, can't say much for the food, it's alright
-New entertainment offerings coming in future years
-No more parade
-pizzafari here has good pizza
Epcot
-Character spot now has an area to meet Baymax
-Maelstorm is closed, it's being replaced with a frozen area, will be opening later this year
-Soarin is currently down for refurb
-Chip & Dale, Aurora & someone else are leaving Epcot next week
-Italy sit down resturants have great pizza, but not traditional
DHS
-Entire new Star Wars area called Launch Bay has opened where the Animation bld used to be, anything in the animation bld is now gone
-Adventure Happens Here dance party currently going on nightly (?)
-Lots of closures all over the park in plans for expanding
-New Force Awakens scenes added to Star Tours
-Nightly Star Wars fireworks
MK
-The entire New Fantasyland Area (maybe just mine train, this is where my years get fuzzy)
-Belle & Ariel have entire lands, great for princess lovers, Gaston meet & greet as well
-New daytime parade, it's incredible, don't miss it with littles
-Move it Shake It parade/dance party has updated
-The dance party in Tomorrowland has the Incredibles now
-Princess Fairytale Hall has opened, one side has Anna & Elsa m&g, the other is Rapunzel & Cinderella (though entertainment is always subject to change) this took place of the Snow White dark ride
-BOG resturant has b,l & d, all are good, if you have BatB lovers it's a don't miss, go for b or L as dinner is much pricier and considered a sit down
-Pecos has an entire new menu, no more burgers or pulled pork, it's got burritos and such now
-Cosmic Rays is your best bet for burgers
-Pizza can be found at Pinnochios
-As pp said, there's an entire new magicband and fast pass stuff you might not have dealt with
Theres a lot of updates in Mk