Call me cautious.
Directors have been handed comic book film reins before who had no more solid proof their resume had them ready for the task than Affleck does.
It's a fine line to walk though. You can look at majority of the successful films and say that a "fit" seemed obvious.
Singer on X-Men seemed good after great character ensemble with Usual Suspects and drama of Apt Pupil.
Branagh on Thor sounded great as soon as I heard it.
Johnston on Captain America.
Whedon on Avengers.
The only three that didn't immediatly sound right and turned out great were Raimi on Spider-man and Favreau on Iron Man. I really didn't look at Nolan's resume and instantly think that the guy who did Memento was a lock to give us great Batman films.
Tim Story on Fantastic Four was a mixed bag.
Ratner on X-Men 3 was questionable.
Mark Johnson on Daredevil and Ghost Rider were mixed results.
So Affleck on a JLA film....I'm iffy on the thought.