Well, I don't really know why, exactly. Like the world that he creates he is a bit elusive to define and pin down. A postructuralist at heart => clearly!!* His writing is infectious and wonderful and full of the sort of energy that grabs you and takes you for a rollicking ride all the way through the book. Following the advice of a friend I have read them according to character stories, rather than the published order of the Discworld Series. This advice proved invaluable as I was never all convinced that Rincewind was worth a book on his own (even with the Luggage) - let alone the first three in the series.
Anyways having plowed through all of the Witches Ones; The Guard Ones (favourites!!); The Gods Ones; Most of the Death Ones; I am resorting to the Rincewind Ones. And they are Good! You see the world is so fabulous and amazing and wonderfully realised that you forget that you might once upon a time disliked the main character. Seriously. So the moral of this story is. Read Terry Pratchett. It will change your life. That you can feel poignancy as well as laughing is very hard to achieve but I can honestly say that until his writing I had never appreciated nor enjoyed such satire of our world.
My Advice: Start with 'Going Postal' and then work your way through the Guards books. (It is such a shame Commander Samual Vimes is married! He's fantastic - and yes, fictional, I do realise!) Then play around. It's such fun. By this stage you will want more and more and won't get confusion burnout by reading from the first book onwards.
So anyways, having wandered around the reasons, and musings I do know one reason why I love Terry Pratchett: because he creates a world so crazy-wonderful that works just fine and it makes ours here feel that it can work just fine too! Stay tuned for more reasons- I have a stack of them beside my bed.
* You know it is getting bad when your essays start creeping into normal writing and conversation... this happens ALL THE TIME with postructuralism. It is everywhere (and nowhere, of course!)