Nigel Planer should have played Rincewind
I love David Jason.. he's a great actor and he was a perfect "Albert" in Hogfather, but he's far too naturally self-confident to play Rincewind (in Colour of Magic). Nigel Planer's nervous wizard Sideney in Hogfather was so much closer to how I would have imagined Rincewind though, shaking with fear and cowardice and having meltdowns about how unfair life is etc.
Also, while I'm nitpicking, the wizards were all far too thin! And Death's voice, although good, needed even more monotonous GRAVITAS (James Earl Jones' voice, with additional special effects perhaps?). Generally the two movies were, in terms of design and imagery very well done; they got the best they could have out of the obviously limited budget, but the slow pacing nearly ruined the movies; the Discworld books are so wonderfully over the top; some over-acting, shouting, mayhem and fast paced scenes would have been somehow more appropriate! As it was, it seemed that the characters were muttering to themselves and generally half asleep rather than going mental as they tend to do in the books.