A very absorbing read, Simon. Two issues, which you touch on, come to mind.

Firstly, language. Linguistic invention is a mainstay of scifi. I read both A Clockwork Orange and 1984 in high school and I think they offer a clue. Doublespeak, "...raising the chocolate ration to twenty grammes" is much easier to grok than Nadsat, which for me was a distruption to the reading experience. Not that linguistic invention can't succeed on the page, both grok and soma have entered the common vernacular, but it can overwhelm the reader. On the big screen, Nadsat adds to the atmosphere and its literal meaning become less important. Perhaps Frank Herbert had an understanding of this by substituting Spice and Dune for Melange and Arrakis throughout the text. "The Melange must flow" doesn't have quite the same impact. For 2001, tetrahedron is a better choice than pyramid, with all its metaphysical allusions yet monolith is better still, which bring me to the second point.

Visualisation. World creation is the other maintstay of scifi. Scifi, unlike its other imaginary brethren, fantasy, is harder to visualise. We all know what a dragon and a knight looks like, right? 1984, with its urban decay, is fairly easy to visualise and thus much harder to make more compelling than the image in our mind's eye. In visual terms, Kubrick's shift to the monolith is inspired, while Lynch's Dune visualising the Atreides Army as a bunch of boy scouts is an epic fail. For Planet of the Apes, the Statue of Liberty, both symbolically and in setting, is far more compelling than the Eiffel Tower of the book or the Lincoln Monument of the Tim Burton version.

So the big screen has the opportunity to make the language more comprehensible and the visuals more compelling than what we can imagine. If it can do both, it will usually be more successful, and more widely consumed, than the book.