Many writers are excellent world builders; it's sort of a requirement in good fantasy literature. Terry Pratchett, however, ranks among the best. Discworld is engrossingly sprawling and feels real — just like in our world, there are laws governing Discworld that make it feel less like pure fantasy and more like a land we all missed out on being born in through sheer chance.
2. Every book can stand alone.
Despite how complex and intricate Discworld is, the books are accessible to any new reader — because you can pick up any of them and dive right in. Imagine trying to do that with Game of Thrones or The Lord of the Rings (both excellent series that I adore).
If you like to have your chronology in order, some devoted readers have created a map that suggests starting points and reading order.
3. A major theme of the books is that knowledge should be accessible, not just confined to the intellectual elite.
Not a new theme, by any means, but explored from every angle in Pratchett's books. His work encourages readers to question social systems and think more broadly and carefully about why certain kinds of intelligence are considered more important than others.
4. All of his books are hysterical.
Lots of great writers can make us laugh. Lots of great writers can make us think. Very few manage both with as much aplomb as Terry Pratchett.
5. The cleverness and skill of his humor.
Being funny is one thing; being able to write about a hundred one-liners per novel is quite another.
6. Engaging, bright prose.
Funny books aren't necessarily memorable; memorable books don't have to be funny. Pratchett's books manage to be both — and to inspire what Stephen King calls the gotta in his readers, as in, "I gotta know what happens next!"
7. Searing social commentary.
Pratchett's books are both straight-up fantasy/adventure, what with the trolls and witches and Death wandering around and all, and biting satire of some of the more ridiculous aspects of our world. In the words of Brandon Sanderson, "Like the best works of fantasy, a journey with his trolls, witches, and crusty night watchmen provokes inspection of our own world. But what other authors do with light allusions, Discworld does with a sledgehammer. And with light allusion too. Then it steals your wallet."
8. Layered, intricate allusions.
Pratchett has a way of making a reference — across literature, philosophy, and religion — that isn't a loss if you don't get it, but greatly enriches the experience of reading his work if you do.
9. Compassionate and complex character development.
Attaching yourself to a fave isn't a bad idea; characters learn, develop, and grow in ways both evil and good across the Discworld novels. Pratchett understands his characters both as individuals and as tools within a larger context of satire and fantasy, and their growth feels organic and honest.
10. Unparalleled craftsmanship.
Pratchett is an extraordinarily skilled writer. His work deals with some hefty, thoughtful stuff, and succeeds in packaging it in such a way that it's accessible, interesting, and funny, without dumbing it down.
11. A profound, lasting effect on the lives of other people.
When Pratchett passed, the internet was flooded with emotional stories of how beloved his work was, how vital it was to peoples' lives, and how much he would be missed. And if that isn't an indicator of a kind of brilliance, what is?