Well, that's definitely my problem with it. It's just not that funny. It's not terrible, just lacks the almost constant belly laughs of every other version I've seen. It's interesting that because of the time compression factor (It tries to fit in bits of the first three books plus several entirely new subplots.) Adams' great funhouse logic of how some of the most complicated aspects of his universe works, is mostly left out. So, instead of comic science fiction, you end up with comic surrealism. I won't give examples here because I don't want to spoil anything.
In an interesting twist, this actually makes the film work MUCH better for folks who have no "Hitchhiker's... " experience. The guy I went with had never read, listened or watched anything from D.N.A. and his mouth was pretty much on the floor at how strange the whole thing seemed. He loved it.

Maybe, in the long run, we have to accept that making a film about popular books can be really thankless.

I think for me the spirit or mood must be the same. I guess that's something that is hard to define, but I'll try.

For example, most people who love Heinlein's book Starship Troopers, the movie was an abomination. But, while I do love the book, I thought the movie captured the essential quality of the metaphor of the post-WWII US government vs. the "red menace" and much else about the books central message, even though the plot was entirely different. So while I'd love to see a "true" adaptation of the novel, I can enjoy the silly film because I can see the connection.

I also tend to believe that when people truly love a book, the creation of both the writer and the reader's imagination when read, it can set unrealistic expectations for a screen adaptation. I was actually very surprised that LOTR was as well-received as it was. But even the most rabid fan of Tolkein's tales knows that it's just not possible, even in three 3-hr.+ films, to get it all.

For HHG, I guess the humor not being up to par would be something that would cause it to fail for me, as it is a key element. But I have a feeling I will enjoy it even if it falls a bit flat.

That's probably more analysis than you were looking for, Bone. But brevity's never been my strong suit....