His ability to write believable characters. Especially how his writing, and the characters, evolve and grow.

Re: the dwarves, Nobby, even Vetinari who started off 2 dimensional and became a respectable, amd necessary, despot.

Edit: the Watch series features a lot of what I described. But I seriously love the Witches as well, Nanny Ogg is my favorite character (and Casanunda is a riot). And over time, we come to understand Granny so well.