Everyone is praising Marvel for establishing a unified film universe to set up for their upcoming Avengers movie, but I see a few problems that a lot of people seem to be overlooking when they start talking about JLA and Avengers.
AVENGERS SET UP:
Iron Man was awesome. It was pretty much the perfect Iron Man movie. I know I personally got stoked when I saw Nick Fury step out and tell Stark that he wanted to speak with him about the Avengers initiative. It was epic, and it was appropriately timed. It was after the credits and the seeding of Agent Coulson throughout the movie was nice too. He wasn't essential to the plot, and he allowed the little tag at the end. It was great!
The Incredible Hulk was pretty good too! It stepped up the awesome where 2003's Hulk dropped the ball. It was an amazing Hulk movie, and it showcased what he could do. The story flowed well. This time, Tony Stark appeared at the end of the film to mention the Avengers, and he came BEFORE the credits. Marvel pushed the Avengers reference from an optional part of the movie to the actual ending of the film.
Iron Man 2 decided to say, "screw it! All we care about is the Avengers now!" Black Widow and Nick Fury became such an important part of the plot that the entire movie would fall apart without them. In fact, without Fury giving Stark the clues (with a few Captain America comics inside) he needed to create the new element he needed for his arc reactor (without which he would've died and the movie would've sucked). Then, after the credits we got a look at MJOLNIR from Thor.
Thor came next, and while I haven't seen the movie, I have seen the trailers - the first few of which revolved around Thor being interrogated by Agent Coulson. I also know that Hawkeye was featured (though from what I understand it was just a quick cameo). I can't comment on this one much as I haven't seen it.
Captain America: The First Avenger comes this month and Marvel has finally said, "ya know what? This whole movie thing? AVENGERS!!!" It seems that Howard Stark plays a vital role in the film, and even the title is a reference to the upcoming Avengers movie.
So what am I getting at? As Marvel nears the release of their Avengers movie, their other films suffer and become more and more reliant on the Avengers. They become little more than 2 hour advertisements for the Avengers. The movies can't stand on their own anymore!
JLA SET UP:
Batman Begins came out and the only reference was one to the follow up film The Dark Knight! A single playing card with a Joker design on it. It wasn't essential to the plot. It just linked the two films together.
The Dark Knight came out a few years later (we're skipping Superman Returns, because it's being rebooted), and there were no references to anything except the next Batman movie, The Dark Knight Rises (there was a quick Catwoman reference early in the film).
The Green Lantern just came out and once again we got no reference to the Justice League. Just a quick look forward to the impending sequel after the credits.
So what does DC seem to be doing? They are creating a universe that is set apart. The Nolanized Batman is grounded in reality, and he wouldn't work in a movie with the other members of the JLA. The Green Lantern would work fine with the other members, but the film had no indication of linking the two films. This idea of a set-apart universe is a great one! This way, the movies all stand alone and can be completely contained. The JLA movie can reference the other films (or not) and not bother the other films. This allows the audience to choose the way they view the films. Marvel is forcing us to accept the Avengers. DC is saying, "take the JLA or not. You decide." DC has enough faith in their properties that they don't feel the need to link them so that they can't stand alone. If the Avengers ends up sucking, then we've been building to a let down. We'd be in for the same thing cinematically that we already got with Smallville's tenth season. Build+build+build=letdown. Now if the JLA movie doesn't work out, then oh well. We'll just see a reboot a few years down the line and the story lines in the other DC movies haven't suffered one bit.