I just think they're trying to jam in too many back stories and people in one movie.
When you look at the Avengers, it worked because all the characters previously had stand alone movies or were well known. Ruffalo didn't do a standalone, but there were previous Hulk movies and TV shows.....
Everyone knows Batman, Superman, Wonder Woman from previous movies, The Flash has a show on TV people would know him from, but Aquaman and Cyborg just seem out of place.
I get that theye founding members of the New 52 Justice Leaque, but they could have used people who might already been known to the viewers or just added one of them to give their back story a little time?