What was so "groundbreaking" about Tron?
I'm taking this from Wiki:
Being one of the first films from a major studio to use computer graphics extensively, Tron has a distinctive visual style.
Also, the way they filmed it and the way it looked as a result was very ground-breaking in itself.
Most of the scenes, backgrounds and visual effects in the film were created using more traditional techniques and a unique process known as "backlit animation". In this process, live-action scenes inside the computer world were filmed in black-and-white on an entirely black set, printed on large-format high-contrast film, then colorized with photographic and rotoscopic techniques to give them a "technological" feel. With multiple layers of high-contrast, large-format positives and negatives, this process required truckloads of sheet film and a workload even greater than that of a conventional cel-animated feature. In addition, the varying quality and age of the film layers caused differing brightness levels for the backlit effects from frame to frame, explaining why glowing outlines and circuit traces tended to flicker in the original film. Due to its difficulty and cost, this process would never be repeated for another feature film.
It's the fact that they filmed on an entirely black set in order to create the effect they wanted. I was watching "The Making of Tron" the other day and a lighting specialist had mentioned that this has never been done before and will probably never be repeated again.