Oh it's a wonderful setting. But my suspicion steps from a review of the umm, GURPS I think, Discworld expansion. While it apparently wasn't bad as it stood, play was a difficult experience as players had to rapidly ajust their Discworld expectations. Apparently there was a general feeling that their unspoken understanding of the setting and characters icluded elements of Pterry's facility with language humour and arcane references which they could not match. Even though it is the result of an irrational expectation, it did lead to a slightly flat play experience.