I've now seen this one about 10 times, so there must be something about it I like!
50's US sci-fi movies were pretty much a mixed bunch: they were either intelligently made and/or thought provoking or cheap and laughable cheese. Forbidden Planet is a bit of both, but in that rarity for the genre, colour.
It also had a head start with the script - although Shakespeare might not have recognised it, it was based on his timeless play and thus guaranteed a certain amount of longevity itself if made well.
It's the story of one mans murderous id artificially magnified infinitely by machines a dead race left switched on 200,000 years before. Along the way the plot bristles with 50's stereotypes and corn so pure you wonder sometimes why you're watching it, but always do. That love triangle thing...yuk! Disney's cartoonery still holds up well, and the cartoon backgrounds straight off the covers of Galaxy magazine etc look good even after 50 years. Robbie driving the car over the desert in the far distance is a hoot though!
All in all, with all faults, the best of its kind and we should be grateful that such a pristine print survives.