Excessive verbiage warning! Shorter me: Terry Pratchett rawks here read his latest book.
Please read this essay on Terry Pratchett and Discworld by Donna Royston.
Just to sum up for those who have (or won't) read the essay, Ms. Royston examines Pratchett's ouevre in depth by considering three books, The Color of Magic, Interesting Times, and Making Money.
At the end of Color of Magic, she argues, Pratchett presents us with this beautiful and epic image of Rincewind dropping off the Disc, over the Rim and into the void. She then says that Pratchett willfully turns away from the terror inherent in that beauty and finds joy and freedom elsewhere in his stories. While he does use fear and tension to great dramatic effect in the later books, he also begins to turn away from the bounding scope of that first book to focus on the infinite space bound within the nutshell of Ankh-Morpork.
Pratchett has said that Rincewind is his least favorite character, simply because Rincewind runs away from everything. While this invariably drops him further in it, to the point that in The Last Hero he comes and volunteers rather than suffer the indignities of the interim, it also has the effect of widening the scope of the story. In hiking his way across the Disc, Rincewind creates lots more land and people that Pratchett has to come up with and detail. Given the physical limits of his books, that pretty much means he's got less time to spend on each location.
We can see him tiring of this forced lack of detail as the books progress. Perhaps what Pratchett likes most of all is coming up with an engaging character and letting him or her loose on the world. He likes to play with the Discworld. Once he got tired of Rincewind, he wrote about Granny and Nanny and Magrat; Vimes and Carrot and Colon and Nobby; Ridcully and Ponder and the Bursar. In his last few books, written in the last 5 years or so, his established characters have started to take a back seat to the new ones. In Monstrous Regiment, Vimes and the Watch hang around the wings, and we see them through Polly's eyes, which amounts to seeing them again for the first time.
In addition, he's limited the scope of his books. A number of novels written since the late 90's have taken place wholly within Ankh-Morpork and its surroundings; these are mostly the Watch novels, although Moist von Lipwig gets as far as the brassica fields near Pseudopolis before his parole golem catches up with him, and no farther. Having so much time to detail the city, then, has allowed it to grow and develop. There's a street map of Ankh-Morpork now. The clacks, which seemed to come out of nowhere between one novel and the next, have revolutionized the city, and also provided one more vector for a threat to its Hegemony. Ankh-Morpork has grown from a parody of Lankhmar and Greyhawk into a pastiche of New York, London, and Hong Kong, even as its citizens become more real and empathetic. The stamp-collecting craze in Going Postal has a visceral enthusiasm behind it, as the Ankh-Morpork mob becomes something other than a threat. Pratchett introduces new characters, no matter how briefly, and makes it easy to connect with the ones you're supposed to be sympathetic with, and to despise the bullies and petty people. Even the time-travel gimmick of Night Watch becomes something more than a mere gimmick, because it ties well-established characters together. For instance, we finally know how Reg Shoe came to have his particular brand of social activism.
In any less capable author's hands, this focusing-in on the lives of a few people might become petty and trite, as their problems and the solutions they find utterly fail to shake the foundations of Pseudopolis. Pratchett has a gift for making us really care about the widow's mites that are at stake in Making Money, and he really makes us root for the little person whose problems may not matter to the Great A'Tuin, but really can spell the end of the world for the people involved.
Pratchett has achieved a level of mastery that few other authors have reached before him. He is a fitting role model and hero for every aspiring writer and dreamer in the world. Hopeful authors could certainly do much worse, and little better, than follow in his footsteps, even though they hardly ever leave the banks of the Ankh anymore.