Agreed with the many opinions that a JLA movie may end up being bad. I think that the DC characters are in many ways are "unknowns" to the casual viewer, that the JLA itself would have to be unexpectedly different in order to be successful.
The JL & JLU animated series was great. The current Young Justice series is rich with character development, political intrigue, and personal conflicts. The question is, how can you have what makes those cartoons successful, condensed into a two-hour movie while introducing new (to casual viewer) characters? I don't think it can be successfully done.
Yes, they WILL have to reboot Nolan's Batman. Nolan's Batman series IMO is the best interpretation of a comic hero to hit the big screen. And his Batman is so fresh in everyone's minds. Unfortunately, Nolan's Batman doesn't fit in the JLA universe. Batman will have to be rebooted because the current viewer will have to be convinced that Nolan's Batman universe can have flying aliens with invincible super powers, and some dude with a ring that can construct anything he can think of. And a hot Amazon chick that has super strength as well... A Martian shape shifter... etc. None of these realistically fit in Nolan's Batman universe.
IMO, in order for a JLA movie to work, WB has to go the Marvel route by introducing the individual characters first. And reboot Batman in a movie which does suggest that his world can have supernatural/extraterrestrial stuff going on.