First of all, to get this out of the way, some people may have (well, this is the internet, so I'm sure some people would have) complained about the changes in the plot -- a relationship between Arthur and Trillian, the Magratheans recreating the Earth at the end of the movie, all that new stuff with Humma Kavula. I want to be clear and say I don't mind that. HHGG has always been a very changeable story; the plot and even characters were different in every iteration. And some people might complain about the happy ending; my response is, what's wrong with a happy ending exactly? Nothing? All right then.
Rather, there are two main problems with the movie. The first is that the pacing was all wrong, being much too quick and seemingly based on chopping dialog at random. HHGG deploys some spectacularily huge concepts but they get short shrift in the film. A good example is Deep Thought. A supercomputer built by hyperintelligent pan-dimensional beings, calculating the Ultimate Answer to Life, the Universe, and Everything? It's a majestic concept, and it has to be presented as a majestic concept so that the punchline -- the answer turns out to be 42 -- is as wonderfully deflating as it is. In the TV show and the radio show, Deep Thought got a good ten minutes of development. Now undoubtedly that could be trimmed a bit, but in the movie the whole thing is just tossed out in about two minutes because the film is so busy rocketing along to the next plot point in a story stuffed to bursting with them, and it loses the impact.
I should mention that you can't just say "well, HHGG fans will get it" because ideally a movie like this is not just made to appeal to HHGG fans; you want to attract people who haven't seen the story before! But are these newcomers going to be impressed by Deep Thought and stunned into laughter by the lameness of the Ultimate Answer, or more just bewildered by this random and underdeveloped plot point? Yes, rather more the second point than the first.
This pacing problem tends to wreck even the smaller jokes and plot elements:
1. Sometimes excising jokes' punchlines and wrecking them completely. The Babel fish Guide entry is a good example: the whole thing works up to the final statement that "The Babel fish, by removing all barriers of communication and understanding between different races, has therefore been responsible for more and bloodier wars than anything else in the history of the Universe." Without that, there's basically no joke in the whole Babel fish sequence at all. And then there's the line in the original versions about how the plans allegedly "on display" for demolishing Arthur Dent's house were in a locked filing cabinet in disused lavatory in a cellar with no lights or stairs, the point being that they damn well weren't on display -- just like the plans for demolishing Earth were "on display" near Alpha Centauri. In the movie, Arthur just says he had to go down to the cellar of the planning office, the viewer might be forgiven for wondering what the big deal is about the display department being in the cellar, and the whole thing falls flat.
2. Other times removing the parts that give additional definition or heart to the Hitchhiker's universe. There's a gag in most of the versions of HHGG where, when Ford and Arthur are in some terrible spot like being stuck underneath an enormous boulder five hundred feet underground with no hope of rescue, Ford announces that the Hitchhiker's Guide is so ultra-complete that it'll have something to say even about the extremely specific crisis they're in. He looks up the extremely specific situation in the Guide, and the Guide's response is something like the following:
What to do when you are stuck underneath an enormous boulder five hundred feet underground with no hope of rescue. Consider how good life has been to you so far. Alternatively, if it hasn't been good, which given your situation seems far more likely, consider how lucky you are that it won't be troubling you much longer.
This isn't just clever dialog; the point being established is that the Guide is ultra-complete, yes, but never actually very helpful. In the movie, that line is deployed without any specificity about the situation, which in turn kills the point it was getting across about the Guide.
3. And other times removing the connective tissue that makes some sense out of what's going on at all. When Arthur and Ford are captured by the Vogon captain, he doesn't tell them he's going to read poetry -- he just launches into his weird monologue. And then when Arthur tries to bluff him and claim he liked the poem, the captain just throws him and Ford off the ship with no further explanation. How is a newcomer supposed to know why any of this is happening? Or consider Magrathea -- when our heroes finally arrive, they are informed that the planet is "closed" and are attacked with missiles. Why is Magrathea closed? Why are they being attacked? In the context of the movie there's no answers to these questions. Too much of the movie is like this, a sequence of disconnected and unjustified vignettes.
The second problem with the movie is the acting -- or, rather, the directing of the actors. I'm on record as being very pleased when I saw the initial casting for the film, and I stand by that; the cast was perfect. But it seems like the actors weren't given any direction and were instead left to fend for themselves and come up with their own idea of the character. This worked okay in some cases; I'm especially thinking of Bill Nighy, who portrayed Slartibartfast as a sort of superbly dedicated and proud state highway worker. Martin Freeman blustered his way through as Arthur Dent, which more or less works given that the movie's Arthur Dent is a much more proactive character than the other versions. After that things kind of go downhill. Sam Rockwell was hilarious to watch as Zaphod Beeblebrox, but it's not clear that most of his seemingly ad-libbed dialog or actions actually fit into the story. Mos Def as Ford was clearly trying hard but he just didn't have anything to work with -- the scene with him and the Point of View Gun gives it away; he doesn't have a point of view! And as for Zooey Deschanel as Trillian, she didn't even seem to taking any of this seriously. Without all the actors working together as a team it just got harder to suspend one's disbelief.
I'll say this, though: whoever did the sets, costumes, and visual effects clearly had a great time. The Vogon fleet/city, the Heart of Gold, the Magrathean factory floor -- it was all so gorgeous and imaginative and there was so much love in all the designs and effects. Even when the actual script was flailing, everything on the screen was a pleasure to look at. That makes up for a lot.
Probably... not enough in the end, though.