There are a number of reasons why an animated Justice League movie is appealing, some of which are fairly time-sensitive and some are not.
Here's the thing: I'm not talking about (yet) another of the DC Universe animated features. Those are nice and all but, frankly, lack the scope of a theatrical feature. While the animation is generally very good and the names they attract are top shelf, the limitations of the 70-minute runtime and the stigma of a direct-to-video format really hobbles the feel.
Also, there's a lack of "realism" to those films, because they look like comic books or traditional animation. I'm talking about doing a full-on, two-hour, CGI rendered or motion-captured film that's in the vein of something Pixar or DreamWorks might put out.
Granted, Warner hasn't done anything like that with any degree of success yet--but it's hard to argue that the properties of the DC Universe aren't begging for a shot at it--and as they've recently started beefing up their animation department in order to meet the changing needs of the theatrical release market, Justice League could be a perfect first film to really giver Warner Animation a new face.
It would also be one of the few areas where Warner could beat Disney to the punch; while they've got a Big Hero 6 animated feature in development, the Marvel Cinematic Universe has been so successful that it's unlikely we'll see a big-budget, high-profile animated film anytime real soon out of the company.
DC, meanwhile, has had the opposite problem; while the DC Universe movies do pretty well, their feature films have been just terrible, Batman notwithstanding, and the Batman films that existed were so aggressively trying to be art films that there was no opportunity to build a universe out of them. Jonah Hex, Catwoman, Green Lantern...these are names that, if they're remembered at all by the mainstream audience, they're remembered as disasters.
Admittedly, turnaround could be a problem; movies on the scale of what Pixar and Dreamworks do rountinely take 3-5 years to complete...but of course, three years is more or less the norm anyway for a film like Justice League. It's been nearly four between Star Trek films.
Why do I think an animated feature could be the way to go?
You want CG? You could get Zack Snyder's owl people on it and have a nearly-photographic film, but one in which tights aren't a challenge and simulating a Boom Tube is scarcely more expensive than showing somebody cooking bacon.
You can cast whomever you want; in animation, it's more common to record either alone, or with a small group...which means there's really no need to worry about trying to wrangle deals to get everyone in the same room, at the same time.
You can have whatever stars you can nab to do the movie, and not particularly have to worry about scheduling. It's also generally cheaper to get people for animated work, as is evidenced by the success of the DC Universe animations, which thrive on low production costs and familiar characters played by recognizable names.
A film like this could be revolutionary, in that it would be something that has never been done before. It would generate a huge amount of heat and interest before the first trailer was ever screened.
As we've learned with the DC Universe animated films, there's an enormous amount of material to mine in the DC Universe. Much of it, though, would either look pretty ridiculous in live action or would have to be changed to accommodate the format. Ask Bryan Singer how many people are still kvetching at him about the stupid black leather suits nearly fifteen years after the release of X-Men.
The suspension of disbelief is bigger in animation than it is in live action films, because there's not nearly as much
And, of course, as alluded to above, there are events that are incredibly hard to do in live-action. They're time-consuming, they're expensive, they're impractial and sometimes you just end up with a shot that doesn't look very realistic. All of that goes out the window when you're dealing with an animated canvas.
It would also streamline the process of developing effects for the film; they're animated like everything else, full stop. No need to figure out how to make the big green hand of light, which is CG on film, carry a practical object over GL's head.
There are dozens more good reasons, certainly...what are yours?