Again and again and again. I hear something different each time.
the fact that Pratchett shows up all the the silly prejudices about women in a humorous but powerful way.
His ability to change his accents an voice makes all the characters come alive.
The whole book told the story of a group of people thrown together and how they come to respect and care deeply for each other--all in preparation for a surprise ending.
It's funny, has good characters, and good action. You don't need to know about the other discworld books to understand this one. The narration is well paced and captures the humor well. The only trouble is that the narrator runs out of unique voices, and starts making them nasally.
Practically Pratchett-Perfect in every way. Time we met an Igorina, too. Satisfying to the socks;)