Alien vs. predator....
It sounds as though they've just thought they can stick two superheroes in a film and get a hit after they saw the avengers to be honest...
There are way too many superhero films right now, and to be honest many of them are starting to be quite shoddy.
Difference here is that there are 70 years worth of stories with these two together out there already, it's not just picking two unrelated movies and throwing them together.
Some people seem to think that Christian Bale is the only actor to ever play Batman. He's not even the 10th. Each version is different, and only one of them was really poor. The Nolan Batman doesn't exist in a world where flying aliens makes any sense, but there are plenty of Batman stories that do.
It's a new Batman. Man of Steel was a new Superman. There will be more versions of both in the future because these characters are now a part of our culture as much as Sherlock Holmes or Robin Hood. Sooner or later, this was going to happen.
What makes it interesting is the fact it comes so soon after Marvel did The Avengers. Marvel played it perfectly. On their own, most of the lead-in Avengers movies were pretty so-so. There certainly wasn't anything to compete with the psychological depth of Batman Begins or its sequels. But then Marvel had to leave their two biggest characters out of the line-up.
And the Justice League concept, well I'm not sure how many more super-heroes the public wants to get to know. Green Lantern recently tried and failed to jump on the bandwagon. It was a mess of a movie. So do you reboot him as well? The Flash? Great character but he's really quick - Superman's really quick - it's hard for him to really stand out in a team movie alongside the big guy. Aquilani is the go-to superhero punchline on Big Bang Theory. Putting them together as a team is a major challenge, especially if you don't have the luxury of leading in with solo movies.
So Superman and Batman. They are the big two of the Super-hero world. The originals. Only Spider-Man comes close in terms of being so purely iconic. There was always something inevitable about this. But it is a hard sell. They can't out-Avengers the Avengers, it needs to find its own tone, hopefully something between the darkness of the Dark Knight trilogy and the fun and wonder of the Christopher Reeve Superman movies. Cast the right Batman, get that chemistry right, find a way to convey the mutual respect (earned over the course of the movie, of course) and show how they complement one another perfectly.
It could work.
EDIT - loving the autocorrect. The "aquaman" I'm talking about here is the actual King of Atlantis super-hero, rather than the equally mysterious Italian midfield maestro.