To Kill A Mockingbird's bravado, so to speak, was to shed light on a really crazy subject at the time of its conception. There's a reason it is so hailed as one of the best book-to-movie renditions out there.
Yes, it is a fairly simple story to follow. No, it wasn't a very basic subject to follow. Especially not in 1960's USA, you know the one where the country was still segregated, and white people (in general) hated black people? It was a touchy subject, and Harper Lee and Robert Mulligan nailed it.