The characters are in costume and in action on virtually every single page from the get-go, and there's plenty of opportunity for merchandising between the heroes and the endless array of robotic bad guys they square off with. Everyone (yes, even Aquaman) gets a chance to shine, and the idea of introducing a new DC cinematic universe with all of the characters together and then spinning them off into their own films certainly must have seemed attractive, even in those pre-Marvel Studios days.
While it's refreshing to see these characters presented pretty much exactly as you would want to see them, with little in the way in deconstructionism or even soul-searching on display, it does make the proceedings feel a little lightweight. Justice League: Mortal sometimes reads more like an extended episode of the Justice League Unlimited animated series. This isn't necessarily a bad thing, but things move along a little too easily considering how many characters need to be introduced.
The film is bookended with a funeral sequence for a hero, although we don't find out who it's for until the end. Once that shocking opening is out of the way, it's made clear that superheroes are already well-established on Earth. In fact, they're so well-established that Wonder Woman is addressing the UN to discuss the fact that humanity (with the help of their superpowered protectors) appears to have achieved world peace. Even Bruce Wayne's faithful aide, Alfred, tells Bruce that crime in Gotham City has been reduced to a "nuisance."
It's an interesting opening gambit, almost like a far-reaching, optimistic version of Watchmen, but it's glossed over so quickly (Wonder Woman's speech to the UN is broadcasting on a TV at a restaurant where Barry Allen and Iris West are eating), and referred to so infrequently afterwards, that it's almost inconsequential. It makes for a nice change of pace from the origin story addiction on display in most superhero movies, and the novelty is more in how these characters get together and interact rather than how they came to be in the first place.
It's never made quite clear how long superheroes have been operating, but I figure five years is a safe bet, especially for Batman, who has probably been operating longer than any of them. The world's superhumans have never teamed up on a large scale, but some of them appear to have met before.
The Flash functions as the point of view character of the film. Despite his great power, he's the joking everyman, constantly in awe of the other heroes around him. It's not necessarily the most in tune with traditional depictions of Barry Allen (although one could argue that Grant Gustin's version of the character could slot right in here), and Barry's sense of humour is similar to how the Flash of the Justice League animated series (although on that show, he's Wally West) was presented. The fact that a seventeen year old Wally West is also hanging around makes things a little distracting, as his personality is almost indistinguishable from that of his Uncle's.
Wonder Woman on the other hand, is the most "outgoing" member of the team. I mean that inasmuch as she has apparently already met/worked with Superman (they're on a first name...not codename basis), Aquaman (who she has some romantic tension with), and possibly Martian Manhunter. She's also the public voice of the metahuman community. When we first meet her, she's addressing the United Nations in a television broadcast, which is a nice way to set up Diana as an ambassador, although little is made of her Amazon background. It's safe to assume that's all in place, though.
In short order we're introduced to the rest of the team once the Martian Manhunter finds himself the victim of a mysterious attack that leaves him in the uncomfortable position of bursting into flames whenever he's exposed to oxygen. As each member of the team goes to his rescue, they each find themselves compromised by nanotechnology that exploits their weaknesses.
Why is this happening? Because Batman's been hacked, and his files on how to take out assorted superhumans if they ever got out of line are now being exploited by Bruce Wayne's buddy Maxwell Lord, who is also playing around with the government's super secret OMAC technology.
How did this happen? Because Talia al Ghul slipped some tracking tech into Batman's shorts during one of their romantic interludes.
Why is that happening? Because Maxwell Lord wants revenge on the world for horrible experiments done to him as a child as part of the OMAC Project, which left him with some low-level psychic abilities. Simple, right?
Eventually, everyone gets back on their feet, they make their peace with Batman despite the fact that he's a pointy-headed, paranoid, fascist dikk, and get ready to take on Maxwell Lord and the OMAC cyborgs...who unfortunately have innocent people inside them. In the course of this, Superman ends up mind-controlled and the team has to deal with a Kryptonian running amok on top of everything else.
Remember what I said about Flash getting the most screen time? Well, he also gets the most dramatic moment. See...remember how I said this is bookended with a funeral?
Barry Allen sacrifices himself to get rid of Maxwell Lord (who has become a cybernetic doomsday device) by basically running so fast he merges with the Speed Force and sucks the giant OMAC into oblivion with him. Ummmm...it actually reads a lot better than I make it sound. Wally West then takes up his Uncle's heroic mantle and joins the team at the end. All the Flash stuff is handled really well throughout the movie, from representations of how Barry sees the world when he's moving at full speed to his relationship with Iris West. His death definitely would have hit an unspoiled audience pretty hard.