The story itself just never manages to be engaging, and there is no arc of suspense. Even in the third novel, which is supposedly about saving the galaxy from great danger, one does not get the feeling that any significant number of things happen that advance the action. Even in those rare cases when they do happen, they don't feel like it - their impact is lost. And the first two books don't even have the advantage of a story arc that could potentially be interesting, they are just one event randomly happening after the other.
Part of the problem is that I find it hard, as a reader, to care about anything. On the one side there is the setting, the world in which the stories take place: our galaxy. It is presented as a supremely cruel, uncaring and depressing place, and thus one would be forgiven to consider Hactar to be the hero of the third novel. The destruction of the randomly genocidal, exploitative, bureaucratic and utterly incompetent galactic civilization as portrayed by Douglas Adams would not appear to be a great loss because one is never given a reason to feel for it nor given a chance to develop empathy for its members, which are all either morons or downright malicious.
On the other side there is the cast. The way to make your reader care about what happens to a character in your story is to make that character likeable. The reader has to identify with them or at least pity them. I find it hard to do so in this case. The main characters of the Hitchhiker's Guide are too flat to identify with them, they are too unlikeable to be pitied, and most of what happens to them is their own fault anyway. Indeed they are so one-dimensional that their entire respective personalities, everything you need to know about them to make sense of their behaviour in the first three books, can be summarized in one sentence each:
Arthur Dent is perennially bewildered about the strangeness of the universe, and his reaction to crises is always to freeze in panic, which makes it hard to sympathize with him. He does not seem to have any characteristics beyond that nor any motivation or goals that have any connection to the story, and as such I am puzzled why so many people think he is the protagonist of anything before the fourth book. He is one of several viewpoint characters and the comic relief, but the protagonist of the first two books is Zaphod, and in the third it is Slartibartfast.
Zaphod Beetlebrox is a self-important, conceited, irresponsible idiot. Ford Prefect is a hedonistic, laid-back, irresponsible idiot. Trillian serves as the only sane person in the ensemble, but is too shallow and self-centered to be likeable. What is more, her supposed intelligence and sanity only make it harder to suspend disbelief about her not doing one week after getting together with Zaphod what she ultimately did in the third book, i.e. leave him in disgust. Marvin is misanthropic and insults everybody.
That's about it; I don't think I have missed any relevant aspect of the personality of any character. Perhaps it works better in the radio show, and maybe the Dirk Gently novels are much better, I wouldn't know. But the comparison that goes "Terry Pratchett is the Douglas Adams of fantasy" seems very unconvincing to me because Terry Pratchett is actually a great storyteller and able to create believable, deep characters one can identify with. They are often basically decent human beings but also realistically selfish; they have understandable motivations that are sometimes short-sighted but well within the parameters of what we should expect of them given their role in society and the knowledge available to them. They would not, for example, have stolen that black spaceship from the parking lot of the restaurant at the end of the universe.
To end on an upbeat note, I consider Last Chance to See one of the best popular science books ever written. That work really shows Douglas Adams at his best, and if you have never read it you should seriously consider doing so.
Rather extensive footnote
*) Interestingly, I have a very similar problem with classical German drama. It goes like this:
- Be forced to read Goethe and Schiller in high school. Be told that they are the greatest German poets and something to be immensely proud of. Put that down to German patriotism.
- Start reading Shakespeare. Be told that he is the greatest poet of all history. Put that down to British patriotism.
- Ca 14 years after high school, read the German classics for the second time. Discover that while Goethe now reads so-so/okay, kinda on the level of Marlowe or Kyd, Schiller reads as a fourth rate hack who would have made a perfect Heimatfilm screenplay writer if he had not been born fifteen decades too early.
- Start thinking that there may be something to the aforementioned claim about Shakespeare if that is the best that your home country can come up with.