I think people went to see it because he is supposed to be an anti-hero, an over the top super power. He is foul mouthed, extremely crude & rude. A parody of super heroes who are a parody to begin with. The brief nudity isn't any worse than things cable has on tv, or in serious, Oscar potential movies.
After seeing so many brutal fights in super person movies, Deadpool's aren't really much different except he takes great delight . The back story kinda explains what he has become. But, what sold me was the romance - yes , sweet & everlasting. Okay I am sentimental. Hope I didn't give away too much