The antithesis of the MacGuffin in my understanding is that the motivating device is of real importance to the central characters. My guess is that the difference would be rescuing the princess because the king tells you to is a MacGuffin, while rescuing one of your characters best friend of family member is not.
I'm sure this is not an either/or issue - the degree to which there is importance in the motivating factor could vary as well.
So - here's my query: To what degree does everyone have MacGuffins or anti-McGuffins in their own games or notable games you've played.
What are your favorite strong motivating factors for the characters (anti-MacGuffins)? For this question, I'm looking for ones where the players really enjoyed the factors involved.