Ranked in these QuestionsQuestion Ranking
Pro Highly intertextual
There is an incredible amount of references to books/films/music/pop culture/historical events and figures/whatever else you can think of. Some of them are quite obvious, some you might get only a few years after reading the book - when you learn some small random tidbit of history or watch a particular scene in the movie. It's not only extremely enjoyable but also makes you feel proud when you suddenly connect the dots and figure out some obscure reference.
Pro A complete universe
The books are (while not exactly based on each other) adding each a new facet to the imagined world of the Discworld.
Pro Funny like hell
Terry Pratchett has a way of putting every day situations into new contexts that makes you laugh.