My first point was that before everything was made by hand, the scenery, settings and everything. The first computer generated effects were horrible because technology is an ever improving thing. And the thing about older Sci-fi is that the ideas were there.
But it's not like miniatures were perfected in the 70's. The models only improved, and the bigatures in Lord of the Rings looks much more convincing compared to a lot of miniatures from the 70's.
It's like @PostIronic and I discussed once, there was something in the water then. Call it hallucinogenic drugs, the moon landing or whatever, in that time period and that time period alone people in art were open to ideas we've had never thought before and probably wont again. Even the somewhat bad-looking Sci-fi movies of the era had character and such unparalleled imagination. Nowadays there's nothing that even tries to think outside the box in the genre, it's action minded, explosions and CGI. Bear in mind i haven't watched Interstellar yet.
It's not that there was something in the water. A big part of it is the Hollywood studio system failing in the early 60's and as a result giving directors more creative freedom in the late 60's and 70's. Furthermore you get new tax laws making it more profitable to invest in films. Without those laws films like
Taxi Driver and
One Flew Over the Cuckoo's Nest probably never would have been made.
At the same Hollywood finally got the younger audience to go to the movies again with relatively cheap movies like
Easy Rider and
The Graduate. Likewise the hippies loved
2001: A Space Odyssey because of the visuals. And once you have a genre hit, other films in the genre are sure to follow. The directors of the 70's just had a lot more creative freedom compared to now. Unfortunately, films like
Heaven's Gate proved that creative freedom could result in a catastrophe, and at the same time movies like
Jaws and
Star Wars proved to be recipes for box office hits. And gone is the creative freedom of the 70's. It's not that the filmmakers of today lack imagination, they just don't have the freedom to do what they want.