Some fans have always insisted that Nobodies do actually have hearts, despite the games claiming the exact opposite. In this game, Xemnas explains that they're half right. Nobodies are indeed created with no hearts, but develop replacements for their hearts over time.
Some fans have speculated that the depths of Sora's heart looks like the Destiny Islands. They were right, as when Riku dives into Sora's heart to wake him up, he finds Xion, Roxas, and Ventus on a perfect replica of the island Sora, Riku, and Kairi visited when they were younger.
Milestone Celebration: Dream Drop Distance is a large part of the series' 10th anniversary celebrations; the logo for it is emblazoned in the top right corner of the games cover, and it also accompanies 358/2 Days and Re:coded in a special anniversary box set. Additionally, a limited edition 3DS console with a unique 3D-inspired design is being released.
Hayden Panettiere has replaced Alyson Stoner as Xion. This one is interesting—obviously the intention is for Kairi and Xion to have the same voice actor, but Panettiere was unavailable for Xion's debut, meaning Stoner is her original actor.
There was originally going to be greater differences between Sora and Riku's stories. Sora was planned to return to Space Paranoids, with only Riku visiting the Grid (there are elements of The Artifact here; note that Sora's plot is concerned with reconnecting with Tron, while Riku's is the one with the condensed Tron: Legacy plot). Additionally, Sora's visit in the Symphony of Sorcery would've been based on Fantasia, while Riku's would've been based on Fantasia 2000.