Interior of the spaceship looks fururistic even to today's eyes
The interior of the spacecraft has aged extremely well. In fact, it hasn't aged at all. Some spaceships of the 1950s and even later look laughably dated by today's standards. Like the one (I forget which one) that had spacecraft passengers seated in folding chairs--they weren't even bolted down, and not a seatbelt in sight. Even early Star Trek sets look, well, old-fashioned.
In contrast, Klaatu's spaceship interior is sleek, beautiful, and so high tech that there's virtually no visible tech at all. This seems to be a civilization that has evolved beyond buttons, handles, levers, and readouts. It's a brilliant and timeless design.