3) The Film Isn’t Tied Down By A Wider Universe
In another case of Deadpool making an advantage of Fox having so little faith in the film, Deadpool isn’t really connected to Fox’s wider superhero universe. There are no references to the Fantastic Four, and Deadpool the character even ridicules the fact that no big-hitters from the X-Men universe appear in his movie (just a fresh version of Colossus and brand new character Negasonic Teenage Warhead, as Wade Wilson’s sidekicks).
This means Deadpool isn’t tied down like the movies in the MCU, none of which – perhaps with the exception of Guardians – are ever really allowed to be self-contained stories. There are gratuitous cameos (Hawkeye bafflingly showing up in Thor), references and in-jokes that take you out of the movie (Cap in Thor 2), while some Marvel movies basically just act as cynical stepping stones to the next film.
Deadpool is weighed down by none of this; probably because Fox didn’t want to associate its other superheroes with Deadpool in case the film failed, but it works well all the same.