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TRON: Legacy appears as a level in Kingdom Hearts 3D, called The Grid; while it's mostly unrelated to "Space Paranoids" from Kingdom Hearts II, there is a minor connection between the two worlds when playing as Sora. Rinzler also serves as a boss in the game.
Development Hell: After years in development hell, the first indication that Disney was serious about creating a sequel was the showing of a "VFX Concept Test" teaser at Comic-Con 2008. The teaser showed the updated light-cycles, and revealed Jeff Bridges as Flynn and Clu. The title revealed was "TR2N", but by the time the teaser was released (officially) online several months later, the title had been changed to "TRON: Legacy".
Likewise, Daft Punk had a lot of questions for the Disney representatives who approached them about the soundtrack. The pair, unsurprisingly, turned out to be raging fanboys who wanted to know if it would be done "right". The Disney execs admitted that they felt like the musicians were interviewing them for the job, not the other way around.
Executive Meddling: Daft Punk's score was heavily altered by Hans Zimmer and his company Remote Control Productions after test audiences had trouble following the music. As a result, it sounds less like Daft Punk and more like a ripoff of Zimmer's own Inception and Brad Fiedel's Terminator.
Name's the Same: People forget that this Clu is not the same Clu from the first movie, but a separate program with a completely different function who is named after Flynn's hacking program from the first movie.
Michael Sheen was so eager to join Legacy, being a huge fan of the original, that his agents actually had to hide his fandom from Disney so the actor wouldn't get lowballed for it.
Similarly, Daft Punk; Word of God states that his initial interview with the duo felt more like the musicians were interviewing him to make sure he was doing right by the TRON name.
Recycled Script: A surprising amount of plot elements from TRON 2.0, the discredited sequel, match the movie. Son of the original leads? Check. Running through cyberspace looking for his dad? Check. Drafted to the lightcycle arena deathmatch and rescued by a mysterious female Program? Check. Flynn gone missing in both timelines and no-one knows where Tron himself vanished to? Check...
Many in-world examples in the Grid. The Light Cycle duels are now in multilevel, obstacle-strewn arenas (with bikes that can move omnidirectionally, slow and even stop), the Recognizers are much more detailed, the Solar Sailers have become trains, and programs have Butterfly Parachutes.
The Grid itself. Back in The Eighties, when Flynn built it, it was a multi-millon-dollar supercomputer that he used to revolutionize all of ENCOM's software, the source of his fortune. 2011? Sam puts the entire thing on a memory card and wears it as a necklace!
Also averted, as the Grid servers have been isolated from the world at large, so no Internet.
At one point, Sam casually mentions WiFi and Kevin replies "What's WiFi?" He's been stuck on the grid since before WiFi became commonplace. However, when Sam explains, he smiles and notes that he thought of it back in 1985.
In addition to the above, improvements in computer technology in the TRON universe, just as in the real world, would make it reasonable to expect that humanoids, vehicles, water, even simulated weather would be better depicted than as seen in the original film. If nothing else, the programs no longer need to wear goofy hats and no longer look facially like something out of a 1920 silent German Expressionist film.