Adaptation Displacement: Most people are familiar with the series as either a five or six book "trilogy", unaware that it was first a radio program. To be absolutely clear:
1. It was a radio series...
2. That got adapted into a book series...
3. Which had a TV show made of it...
4. That had a text-based computer game made of it...
5. Which, many, many years later, had a movie made of it...
6. And we can only assume it will eventually be available in pill form at some point.
Angst? What Angst?: Trillian barely notices when her home planet is blown up and billions are killed. Dent has the decency to be shocked for a few minutes, though once he realizes that he can't quite wrap his head around the magnitude of the loss he starts going into shock.
The film version of Trillian doesn't even find out until late in the story thanks to Zaphod. When she find out, she goes ballistic.
Common Knowledge: 42 is not "The Meaning of Life", it's "The Answer to the Ultimate Question of Life, the Universe and Everything." The reason it seems so random and nonsensical is because it's only an Answer, and no one actually knows what the Question is.
The 6" single of the TV theme music had a rather catchy song on the B-side called "Only The End Of The World Again", credited to Disaster Area.note According to h2g2, Douglas Adams played guitar on it.
"Ten out of ten for style [or X], but minus several million for good thinking, huh?"
Mis-blamed: Many people actually cry They Changed It, Now It Sucks to the various adaptation(s) because they're "not like the book". Adams wanted the various formats to diverge as soon as possible, and succeeded.
Hilarious in Hindsight: While the Shoe Event Horizon was funny back then, the rise of the Starbucks coffee chain means the world seems to be heading the same way in Real Life.
Student: Shoe shops have to sell more shoes, so they sell shoes so bad they either hurt the feet or fall apart. So people have to buy more shoes. Which means more shoe shops. Eventually it becomes economically impossible to build anything but shoe shops; the whole economy overbalances! Famine, collapse, and ruin!
Similarly, the joke about how humans are so primitive they think digital watches are neat was a jab at a fad of the time, which as all fads gradually died out (with cellular phones killing watches in particular). Then smart watches came along...
The most egregious being Zaphod's second head, a mechanical prop which barely functioned and veered right into the Uncanny Valley. They tried to cover for it by his first head telling the second to "go back to sleep."
WTH, Casting Agency?: Trillian's accent. Seriously, what the Belgium was that all about? Made even stranger by the fact that she could do a perfectly passable English accent, as seen on a few outtakes. Apparently she asked Douglas Adams if he wanted an English accent, and he was so happy with her casting in the first place that he said no, she should use her normal voice. He came to regret this, in part because he realised it wasn't a particularly flattering thing to say to an actress.
Death of the Author: Many people who are critical of the Romantic Plot Tumor are surprised to find that it was in Douglas Adams' pre-mortem draft of the script and that he is not, in fact, rolling in his grave over it. Upon a little more thought, most of them conclude that that doesn't make it any better.
Ear Worm: "So long and thanks for all the fish, so sad that it had come to this, we tried to warn you all but oh dear..."
Mis-blamed: Many things the fans complained about were Adams' intention from when he first outlined this adaptation - fans should remember that he tried to work in new bits into every new Hitchhiker's adaptation - and much of the script was written by him.
Older Than They Think: Some people think that the movie ripped off the name "Babel Fish" from the now-defunct translation website, completely forgetting of course, that it is in fact the other way round considering the source material. The name itself, meanwhile, is in turn based on the biblical story of the "Tower of Babel".
One-Scene Wonder: Humma Kavula. Whether you approve of his addition to the story or not, there's no denying that John Malkovich (with the help of the special effects team) makes him a memorable character.
Romantic Plot Tumor: This very movie features a fairly obvious example of this trope, between Arthur and Trillian. The "original" source materials (book, TV and radio series) all handled their past differently, but agreed that Arthur had been briefly interested in Trillian during a single superficial encounter in the past; when he re-encounters her during the story, he displays jealousy at a few points, but not much more than that. By comparison, the movie version features an Arthur who is desperately pining over Trillian, who could have been his one true love had he not been afraid to pursue her, and he spends most of the movie time thinking about, worrying about or focusing on her. This was deliberately inserted by Douglas Adams when drafting the movie, before his death, to increase studio interest and audience acceptance of the movie.
Actually, he tried to work it into the television adaptation, but the chemistry wasn't there between the actors.
Which is kind of the joke. Ford Prefect's half-arsed attempts to blend in and the total failure of British humanity to notice anything incongruous about him has always been a part of Ford's story (just look at his name) and one the movie merely played up a bit more.