This is all based on my personal speculations, opinions and beliefs based on things that I've pieced together after reading a fair amount of comic books and watching movies over the past couple years. A lot of this is based upon information found in the comics so a lot of potential SPOILERS !!
Hope you enjoy reading this as much as I did writing it!
The New Warriors, a group of teenage super heroes, were on a reality TV show chasing down villains, in a hope to help raise ratings, tried to apprehend Nitro. The fight, following the failed capture of the villains involving Nitro and Speedball, ended with Nitro exploding, killing himself and 600 civilians (and a dog) while injuring 60 other civilians. The explosion also blew up the neighborhood elementary school.
The prior events caused the relationship between superheroes and the general public to plummet, leading to the beatings of superheroes when they were seen in public, such as Johnny Storm when he went to the club with his girlfriend and had to be hospitalized due to the assault by people outside the club. These occurrences with the addition of the Nitro incident leads to the adoption of...
The Registration Act
The Registration Act, which is the cause of division between the superheroes in the comics, has already been a pivotal point in the MCU during Agents of S.H.I.E.L.D., in which the Inhumans were being forced to register their powers on an index for S.H.I.E.L.D. The Registration Act in the Civil War Comic Event legally forced super powered people to register their powers with the United States Government.
The Registration Act is what started the division between Skye & Agent Couslon's "S.H.I.E.L.D.", who is no longer dead, and the rest of S.H.I.E.L.D. The Act would also be able to reintroduce Coulson into the films because of his new position as the new Director of S.H.I.E.L.D. As well as tying into Agents of S.H.I.E.L.D., The Registration Act would be the stake that is driven between Cap and Iron Man, as seen in the Civil War Comic Event.
Stark VS. Rogers
It's very clear that Iron Man and Cap are feuding in this film (if not, please look at the image above). Although I believe that the most important thing about the opposing Avengers is the platforms that they built their teams upon. Tony Stark is the face of the campaign for the Act while Captain America opposes him and decides to create his own band of heroes called the Secret Avengers.
Stark's platform is primarily based on redeeming the superheroes reputation and becoming public servants. But Tony being Tony is willing to do anything to get the upper hand or to succeed, including recruiting supervillians as comrades to battle against his former partners; this will be interesting to see in the film, seeing how Baron Zemo has been cast and he rivals Captain America.
While Stark has his never lose mentality, Cap hasn't lost his "never say die" mentality and in doing so he is willing to continue being a vigilante and recruits other lesser known heroes such as Daredevil, Luke Cage, and Goliath. In doing so, they are automatically America's Most Wanted and being hunted by S.H.I.E.L.D.
Even though this is our first glimpse at the new web-slinging fan favorite, there was a major hint to his existence at the end of Ant Man. This bomb of an Easter egg was dropped when Luis, portrayed by Michael Peña, told his final he-said-she-said story, which touch based on Spiderman in the slightest bit. This lets us know that at least Spiderman is somewhere in New York currently and this also confirms that we will not get an origins story for Spiderman in 2017, in order to be chronological in line with the MCU.
Tom Holland will be on screen as his first role as Spiderman next sumer. This will be interesting because it is rumored that he will have a small cameo, but he was a deciding member of the Team Stark in the comics; in fact, he was the first character to switch sides. But he also was publicly unmasked at his own will while on the same side as Stark; this is detrimental to seeing how Spidey has kept his life as Peter Parker so well under wraps for so long. Though Spidey won't reveal his alter ego during his first role in the MCU he will definitely be a key component in the movie.