Skillful writing technique
He was able to tell a story slowly without being boring for one minute. A master of the slow-burn joke you should have seen coming-up two pages ago, so when the punch-line comes you're ashamed at not being alert as well as tickled by the way he's strung you along. The same goes for parodies, satires and turning what's un-obvious into 'see it was there in front of you all along -- Now do you understand'. A classic example of Pratchet's humanism is when (amongst other minorities being exploited and attacked, Dwarves, Golems, -- the usual stuff) there's a female werewolf major character in a policeMANs role. Police 'dogs' fine but women! Very unsettling. Dwarves have a venerated 'home' back in the hills but where is her 'home' in all senses?