It seems to me that I have started reading Pratchett out of order. The first one I read, which left me impressed but wanting more, seems to have been later than this. This one appears to be the book which let Pratchett quit his day job, and by many standards probably contains more interesting twists than I give it credit for, but without the leisure of fulltime writing, he seems to have written something slightly less thick with gimmicks and humanity.
This one got noticeably better towards the end, but it was nothing but duty which made me keep going. Not sure who to ask about something richer, without pure gimmick from Pratchett.