Rincewind has always been far younger than David Jason in my minds eye. Especially in the first books which are so many years before the later ones. Nigel Planer with ginger hair would have been a better choice, but he wouldn't have pulled in an extra 1.5 million viewers with his name stamped all over the place.
Once they get around to Guards Guards/Men At Arms/Feet Of Clay/Jingo/The Fifth Elephant/Night Watch (but not Thud!). That'll be when the big fucking ace kicks off. Although maybe the Watch/Vimes novels don't lend themselves to TV adaptation as well as the more grandly magical ones. Small Gods, Pyramids and Moving Pictures would all be good in that regard, plus they're sort of standalonish.
Who would play Vimes is the question? OH! WAIT! They'll get David Jason to do it, no worries.