Mark Hamill's appearance in the Star Wars Holiday Special is the perfect example of why it turned out to be what it is today. The dialogue isn't at all like Luke's in Star Wars. Mark Hamill is a great actor, and the scenes with him aren't bad because of him. It's just that the dialogue was bad, and the lines spoken were never something that Luke would ever say, so it is as if the person who is doing Luke's part isn't really Mark Hamill. Luke's way of expressing overconcern for Han and the Wookiees doesn't really sound like Luke. On the other hand, the cartoon does have Luke's magic that was present in the movies, which is more proof that the cartoon is the best part of the show.
I picked up this picture of Mark Hamill in the Holiday Special at a convention, and sent it to Mark Hamill to sign it: