5. Hit the Ground Running
One reason The Avengers' story felt so taught and well-paced is that Whedon established the film's main plot and villain in the opening scene. The film's inciting incident - the opening of the Tesseract portal and the arrival of Loki - literally occurs in the first five minutes. Only after this crisis is established do we check in on each of the individual characters and get the "assemble the team" moment, which itself doesn't last very long. All throughout the film, Whedon employs his screenwriting expertise and ensures that the story keeps moving along, never letting things settle down for too long. Of course, he had the benefit of having the back stories of most of the characters established in previous films - the only new character, Hawkeye, had his own back story swiftly integrated into The Avengers, without too much fanfare. Nolan and Snyder would be wise to take a page out of Whedon's book and forgo any embellishment of the formation of the Justice League, explaining character origins, or anything of the sort. Set up our crisis early and hit the ground running - the audience is smart enough (and pop culture-savvy enough, by this point) to fill in the blanks and apply their own prior knowledge to the characters and the situations to keep up with the story.