It's worth noting that Disney Blu-rays all have amazing picture quality (this includes all Pixar films, e.g. Ratatouille, Cars, Toy Story), this even includes TV shows like Lost, who's parent company is Disney. They really know what they're doing with Blu-rays, unlike some studios, though I'm sure they'll all learn eventually.
Old films CAN look amazing in HD but only if they've had a good remastering/restoring. Otherwise they can just look old and grainy.