The original only had about 15 - 20 minutes of computer animation. All of the other computer-like effects was back-lit animation, painstakingly drawn by huge teams of animators. Actors were filmed on an all-black set in black and white on a large frame film format and then animation was hand-drawn onto the film negatives. The process required so much film that they couldn't control film quality, which is why all the back-lit animation (i.e., virtually everything you see other than the actors' faces and bodies) has a flickering quality to it. It's kind of a cool effect, but it wasn't intentional. The animation effort was so huge, that they had trouble coordinating the color themes among the animation teams. That's why the colors of the good guys, bad guys and even some of the actors from scene to scene are not consistent.
I know all this because my dad's 15 minutes of fame were from when he was the first person on the planet to officially announce that Tron sucked monkey balls, causing Disney stock to tumble around 5%. He got his picture in Time magazine and Roger Ebert wrote a scathing article about him, saying that he wasn't a real film critic (he was an entertainment industry analyst), that Tron actually had some merit and that my Dad probably gave the bad review so that he could manipulate Disney stock to commit securities fraud.
Actually, he didn't say that Tron sucked monkey balls. What he said was that it was disjointed, confusing and that all of the other analysts in the room were coughing and talking and asking "wtf?" throughout the movie. Disney had hyped up Tron so much that the stock price was trading at an then-unheard of super high P-E ratio for a movie studio. Disney was stuck in a rut of just making Cinderella-type animation and dumb kids movies. Kids in the 80's were too hip for that noise and Disney needed a miracle. Tron was going to bring them out of the 50's and show the world that they could hang with the 80's - plus Tron was going to be so good that it was going to be like Star Wars and Close Encounters combined.
So when my dad announced in the Wall Street Journal that the movie kinda sucked and was not going to be the super blockbuster promised, Disney stock tumbled. They had to halt trading and everything. His announcement came before film critics like Roger Ebert could say anything, so they were pissed.
Cool story.
