Just because a lot of people didn't see every one of them doesn't mean anything. The fact that everybody simply knew each character had a solo film and that The Avengers was a team-up of them all is the main reason it was so hyped and had so much build up. The movie being able to live up to the hype was a miracle and is what made it so big. Without lead-ins a Justice League movie simply isn't going to garner the same amount of hype and even if the film is just as good or a little better, without some kind of gimmick it's most likely not going to be anywhere near as successful. Henry Cavill being Superman in a Justice League movie is all the movie has going for it in that regard unless we see Ryan play Green Lantern again. People wont take too kindly to a new Batman unless he ends up pulling a Mark Ruffalo and end up being the best Batman, which is what WB should be putting an extreme amount of focus on.