Granny Weatherwax was great in her previous outing in "Equal Rites", but it's the bickering trio of Granny, Nanny and Magrat, introduced in this book for the first time, that really bring the Witches stories to life.

This book also introduces a sense of progress and dynamism to the Discworld that is largely absent in the previous books, but will become a staple as the series progresses. Previous books have high stakes sometimes, but don't ultimately alter the status quo - Equal Rites did not end with women being accepted as wizards, everything is restored with a snap of the fingers at the end of Sourcery, etc. In Wyrd Sisters we see multiple characters breaking out of their rigid, pre-ordained roles, and a sort of renaissance taking root in the founding of the Dysk theatre. Though it seems almost incidental to the plot, it leaves the world altered.