You may be thinking I'm contradicting an earlier post of mine here. There I was talking about how Marvel needed to make the public consider Iron Man, Thor and Captain America to be big deals so that their being in a film together could be a huge deal. DC doesn't need to do that, just as the new Spiderman doesn't need a solo film for him being in Civil War to be a big deal. Audiences will care about Spiderman being in a film with the MCU heroes regardless of it not being the same Spiderman they already know.
Here I'm talking about story, if the story and/or plot of Avengers had needed you to have seen those prior films to get what's going on it would have been a bad film, if that's how you write connected stories you should be writing a TV show, not blockbuster motion pictures. Whedon had already directed both and understood the difference.
Rarely you can make 2 or 3 films that feel like one big film, like LOTR or The Hobbit though I feel they overdid it with The Hobbit, But those are very special cases, and LOTR was already like that in The Books, it was 1 giant book published as 3, why I disagree with those who feel it should have been 6 movies based on the 6 books the books are divided into, none of those would have felt like a satisfying movie to me.
The trend of splitting the last films of these YA Novel based franchises in two I feel hurts those films.
The Avengers drew on events that came before, but it also tells us in that film everything we need to know. We don't have a movie depicting the events with the "Mad King" referenced back to constantly on Game of Thrones, if the prior MCU films hadn't existed, those references would be the same as that.
Having seen the other films adds to Avengers enjoyment like reading The Hobbit and The Silmarilion adds to Lord of The Rings, but you do not actually need to know those stories to follow LOTR, you don't even actually need the information from the Appendices, they're just a fun bonus.
Star Wars Episode IV: A New Hope actually was a sequel made without the prior films, and we followed it just fine, the Prequels existence now adds depth to the OT films allowing me to enjoy them more as a Prequel fan then I ever did before. But they worked on their own. If The Force Awakens is written in a way where you won't understand it if you hadn't seen the prior films, even just the OT, it will fail.
So when people act like Batman V Superman or Justice League can't work without establishing the characters first, my thought is, no you're an idiot. Whether or not the characters were established first, if you need to have seen other films to follow those films or get who the characters in them are, or why they're doing what they're doing, they would be poorly made films.
You do not need to have seen Batman Begins to understand The Dark Knight, in fact TDK doesn't even really draw on the events of Begins at all. Rises does draw on the events of both prior films, but it still tells us on it's own everything we need to know.
A number of animated Justice League and Superman/Batman films have worked without any shared continuity to solo adventures. And they have less screen-time to work with. Some but not all recycle voice actors already affiliated with those characters. That however is a superficial connection that could have gone either way. That is also how I refute the "too many characters" nonsense.
BvS and JLA better be the same way, or else I will dislike them regardless of whether or not the films explaining what I don't understand exist. Same goes for future MCU films, they better not get carried away with what a shared universe means.
Even in the Comics themselves, many hate on Infinite Crisis for how it feels like you need to read 20 other story-lines to know what's going on. I love Infinite Crisis but I feel it's dependence on the other story-lines is overstated (I've only read not even half of it's tie ins and understand it just fine), plus comics are by nature a more blatantly serialized medium then film, like TV shows are. But still that complaint exists, and I wouldn't want a film version of Infinite Crisis to be done the same way. Civil War was in many ways Marvel's Infinite Crisis, as is Infinity War, so they better be careful.