Prequels
Generally, I believe that prequels are, for the most part, unnecessary. Any competent film explains enough back story to fill in the gaps for the audience without treating the viewer as a child needing to be led by the hand and explain everything in minute detail.
A couple of examples are "Hannibal Rising" and Rob Zombie's "Halloween". In the case of the first film, there was no reason to over explain Lecter's history and in the film, turning him into a sympathetic hero.
I did not need more back story on Michael Myers. In the original movie, the idea that he might be an indestructible monster was better than trying to give him a sympathetic back story.