It’s funny how a work of art can strike two people so very differently, because I never found Douglas Adams to be funny at all. Clever, yes. Very, very clever. I’d often read something of his and think, “Well, that was pretty witty.” But I almost never laughed.
Pratchett, on the other hand, generally has at least two moments per book where I have to stop and put it down, I’m laughing so hard. This in addition to the times where I chuckle audibly, or smile to myself, or just think, “Well, that was pretty witty.” And on top of that, he has engaging characters, fully realized plots, and something to say that’s genuinely thought-provoking, three things Douglas Adams never managed to pull off (exception: Last Chance to See was genuinely thought provoking, and had engaging characters. But then, it was non-fiction, and one of those characters was Douglas Adams himself. It was still far and away the best thing he ever wrote.)
As to the OP, I do know what you mean. Pratchett has a particular style where he’ll open a book with a scene or a character, then not return to it until half way through the novel. Personally, I like it, because it always gives me a good “A-ha!” moment. His latest, Going Postal, has three:
(Really very mild spoilers follow, but boxed just in case: )
[spoiler]The books opens with a description of “the flotillas of the dead,” an armada of sunken ships that is continuously swept through the ocean depths by a combination of deep tides and the graduated density of water as you get deeper and deeper in the ocean. He describes an anchor falling off one of these fleets and striking the ocean floor, where it’s noticed only by some mysterious fellow identified only as Anghammarad, and notes that it’s the only really interesting thing Anghammarad has seen in nine thousand years.
The next chapter describes someone working on a clacks tower (a system of semaphores) named John Dearheart, who falls to his death under mysterious circumstances.
The third chapter introduces the protagonist.
It’s not until about a quarter of the way into the book that you get the faintest idea of who Dearheart was. It’s not until half way through the book that you find out who Anghammarad is, and why he spent nine thousand years on the bottom of the ocean. The flotillas of the dead are never mentioned again, and yet they represent the central metaphor of the entire novel.[/spoiler]
Personally, I loved the way he doles out the revelations, and relies on the reader to make connections, but I can understand why others would find it frustrating.