I've got a copy of the GURPS Discworld book, and I have to say I didn't like it. It made the classic mistake of trying to tie everything down to game rules, when some of the things that happened in the books aren't definable in that way. Any other gaming system would almost certainly fall into the same trap.
It was things like the increasing powers of Granny Weatherwax that really didn't add up - her ability not to get cut by a sword is not based on her 'casting a spell' beforehand. She is simply a very powerful witch who didn't have time to be cut then, and chose to be cut later instead. Not something that can be put into game terms easily.
In addition, the thought of powergamers and munchkins (I refer only to the people who do this relentlessly with no sense of the fun of role playing) getting to run around my favourite fantasy setting is not a good one.