DC Comics' premiere superhero team (featuring the likes of Batman, Superman and Wonder Woman) has been a dream of Warner Bros. since before The Dark Knight was even released and The Avengers was but a speck in Marvel's eye. Plans for a George Miller directed picture fell apart after some serious development, including a lot of preliminary casting, and then things went quiet for a long time.
The Avengers clearly changed all that. Warner Bros' Green Lantern was a bomb, the future of Man of Steel is still uncertain, and the studio needed a major release to both compete with Marvel's monster franchise and replace their big money-makers like Harry Potter and The Dark Knight. If the movie proves successful it could be used as a launching pad for other franchises.
Basically doing The Avengers back-asswards.
Affleck is one of Warner's golden boys after The Town provided the studio with some serious box office and critical success, and he has recently been offered major tent poles like Gangster Squad, Man of Steel and The Stand.
The Gone Baby Gone director seems more interested in making films that he can write and/or star, so it seems unlikely that he would find a suitable role in the Justice League cast (although he has played Superman, of sorts, in Hollywoodland) and he is already attached to The Stand which is still moving along slowly. This could very easily join Gangster Squad and Man of Steel, with the gig moving down the Warner Bros. totem pole until it finds the right talent.
Would Affleck, a director so focused on establishing his reputation as a powerhouse prestige director, really give up several years of potential work to develop and shoot a massive blockbuster that he has no personal stake in? I'm not so sure, although I would love to see him try a film of that scale one day.
In reality, this seems more like a case of Warner Bros. wanting to keep one of their MVPs happy and demonstrating his value to them by offering him their most lucrative properties. A gesture of good will to ensure the studio stays in the Ben Affleck business.
Whether he chooses Justice League, The Stand, or another small awards season favourite, you can be sure that Ben Affleck will rock it. We are a long way from Gigli, people.