I'm not saying it's fantastic literature, but Marvel Comics have been addressing issues that mainstream movies and books did not touch during the 70s, 80s, and 90th. They touched on racism, people with physical disabilities, and mental issues. It says something when they intentionally make the most powerful mutant wheelchair-bound. Stan Lee has said repeatedly that he wanted to touch as wide a base as possible but especially the young generation about how everyone should be treated with dignity and respect. Also, unlike DC, Marvel made the vast majority of their heros with big flaws that the general public could relate to. It also wasn't just one line of books, it was over the majority of them on a monthly basis. There's a lot to be said for that.