Read: Equal Rites
First, it's plot is driven by a completely new cast, introducing the magical professions. Second, it addresses – in its own particular way – a pressing societal problem, gender differences and discrimination in the professional world starting with vocational choice and training. Third, the rhetoric changed. It is now more subtle, inducing every once in a while a quiet chuckle. Even the pace and structure changed. The story line is more linear, different plot elements build on each other and are more interwoven than before.
I consider this the real take-off of the Discworld series. From now on, Pratchett successfully published most years two or even more Discworld novels. Only recently he slowed down a bit.