Pratchett's writing drips with personality and periodically manages a genuine joke, but far and away the best part of the book is not the writing itself but the subject. The Discworld, it's zany cosmology, intricate but inconsistent, irrational but highly logical, is incredibly imaginative and yet uncannily familiar. Pratchett revels in the act of world-building with the same gleeful animation of a child constructing a universe out of building blocks, and it shows. Imagine a more spectacular, if less profound, Bill Watterson working exclusively in the domain of fantasy/sci-fi parody. Various mythological tropes and satires of our own world mingle and cause a chain reaction.
Prachett does slip into rather unoriginal parody on occasion, and the writing has (more than) the occasional hiccup. But in Discworld, that hardly seems to matter.
