I mentioned this in another comment below about Citizen Kane, but a big reason these hugely known "great" movies don't standup today is explained in the TV Tropes page about why Seinfeld is Unfunny - basically that so many pieces of art were so revolutionary at the time, they they have been endless copied and reiterated over and over, so that modern audiences seeing the original piece of art don't see it as anything special.

2001 A Space Odyssey was specially called out as an example:

2001: A Space Odyssey: Similar to Jaws 1, the so awesome, but now sadly so clichéd uses of "Also sprach Zarathustra".

• One would be hard-pressed to find a scene from any Stanley Kubrick film that hasn't been parodied/homaged to death.

• The famous "Star Gate" sequence, in which brilliant colors flash past the screen as the main character travels deep into space, required some extremely tricky cinematography and caused jaws to drop when the film was released in 1968. Thanks to the incredible advances in special effects since then, modern audiences often find the scene ordinary.

• Other purely FX scenes, like the docking sequence early in the film, had audiences riveted. By today's standards, they're downright boring.