I've talked about this before in other areas, and I believe I did some talking about it a couple of years ago in my blogging. Here's the biggest sins DC has committed as far as the extended universe:
-Separate TV and Movie universes - The various DC shows on the CW are really good. Sure, they're flawed, but I'm coming at it as a fanboy who has read comics his entire life. But I'm happy with what they are doing with them. These characters in these incarnations should have been incorporated into the movie franchises.
- Too damned serious! - Yes, I liked Nolan's take on the Dark Knight trilogy. But then Snyder made MAN OF STEEL in the same vein, and then kept it up for BVS. Not everything needs to be dark and gritty. And quit hiring Goyer to write your movies. Yes, he knows his comics, but he just takes numerous scenarios from the books and throws them into a mishmash which is usually a mess.
-Trying too much too fast - Marvel did the right thing and set up a movie universe slowly, and incorporated many characters before bringing out AVENGERS. DC wanted that type of box office, and tried too hard to get things set up for a JUSTICE LEAGUE movie. Unfortunately, everything has been rushed since, with shoehorned cameos from future heroes into BVS.
-Missed opportunities - After Marvel released CAPTAIN AMERICA, DC would have had the perfect opportunity to do a good WWII comics film. DC could have gone with SGT. ROCK, UNKNOWN SOLDIER, HAUNTED TANK, or THE BLACKHAWKS. In fact, had they done them separately, they could have eventually done a big team-up film. They were so concerned with getting to JUSTICE LEAGUE, that they missed other great opportunities for films.