WoT was the first series for me that made magic measurable. Before that one knew about sorcerers who were considered to be great and dangerous, but you didn't really know how they compared to other magicians.

WoT provides a measurement. Alanna has a power of 1, and Moiraine 2... therefore Moiraine is twice as strong a witch as Alanna. Its fantastic. Of course, Talents and skill and other factors get in the way, but it was wonderful to see it put on paper.

Other authors who have done this to some degree are Tolkein (he directly compared the power of several Valar and Maia in detail) and Rowling (she mentioned who the great witches and wizards were - she even provided proportion e.g. Voldemort fighting three wizards at once).