Warner Bros. could be doing a very smart thing by getting Ben Affleck to direct - they would be maintaining a point-of-difference from Marvel films, which is what they need if they really want Justice League to be successful.
If the general market perceives it as an Avengers clone (even though all good comic book fans know it isn't), it just won't be as successful. It has to have a different style. I'm not saying it has to be Nolan-like realism (that would be a little foolish given the characters), but something more like what Man of Steel seems to be going for.
A little bit less popcorn, a little bit more gravitas.
While The Town and Gone Baby Gone are excellent films, I don't think Affleck would be silly enough to make the style of JL exactly like those. What they do show us is that he can do characterisation and tension well.