They left in all the set-ups to the jokes, but took away all the punchlines.
For example, the Babel fish. The reason why you explain what the Babel Fish is and why it can excrete languages telepathicly is so you can go on to explain the non-existence of God.
The film says "this is the Babel Fish, it translates things", and then moves on. So there's no joke.
That plus doing very stupid things with the story, like going to a random planet to meet John Malkovitch so he can give them co-ordinates fore the infinite improbability drive, which basically renders the entire point of having an ui. The infinite improbablity drive, in story terms, is a way for the plot to get from point A to point B quickly and easily without needed waffle about how you get from point A to point B. You want to go to a long-lost panet that no one has been to for ages? Well the infinate improbably drive does it instantly, without them having to do needness storytelling of how they find a long-lost panet that no one has been to for ages. They just get there so the story can continue.
The film fucks this all up and we get a needless side-story about that planet and John Malkovitch and Zaphod's head (ZAPHOD HAS TWO HEADS. TWO. NEXT TO EACH OTHER. IF YOU'RE GOING TO TO HITCHHICKERS, YOU HAVE TO DO THAT. THE BBC SERIES DID IT FOR FUCKS SAKE).
AND by cutting out lot of the actual things you need to know to understand the story, a lot of the things go unnoticed. Zaphod (or was it Ford?) wears a towel over his head when going up to the Ravenous Bugblatter Beast of Trall on the needless sidequest to rescue Trillian. WHY is he doing it? It's not explained in the film (of course in the books and radio series it's explained that the Ravenous Bugblatter Beat of Trall is so stupid that it thinks if you can't see it, it can't see you). So people who don't know the books miss the joke, and people who do know the books wonder why it's there at all.
I know every version is different.
I know Adams apparently worked on the script (although it was rewritten after he died of course).
But mostly the film entire missed the point of the books and radio series and TV series.
Oh and Mos Def was terrible as Ford.
Oh and the Restaurant at the end of the Universe is at the end OF TIME on OF SPACE you can't go the WRONG DIRECTION to it FUCK YOU FILM.
Oh and Simon Jones is Arthur Dent.
(sorry for the rant. I didn't like the film.)