They are all important. DJ Contra as superman? Armie Hammer as Batman? Armie Hammer could work as supes from a manip I saw, but Contra? No way. I like what marvel is doing with their set up to the avengers movie. They have contracts stating all the actors in their own movie (Downey, Norton) have to reprise their roles in the avengers movie. That's cool, but the budget will be very high. So them going the alternative route and hiring unknowns is a reasonable move.
However, just because they are casting unknowns doesn't mean they can pick anybody. I've looked at the actors up for the roles and so far, I'm only happy with GL. Flash is ok, but I can't see him as Barry. Maybe Bart Allen, but not Barry Allen. Megan Gale as Wonder Woman? I just can't see it, maybe when they release a trailer in 09, but right now I can't see it. Hammer can pull off a good superman from the look of that manip. He has the right look and height. As for the rest of the rumored cast I don't know. Cabrera as aquaman? It's Jessica Alba from FF4 all over again.
For a JLA movie to be done, everything has to be good. Not just the JLA the same for any comic movie, or better yet all movies in general. The Screen play has to be good for the director to bring it to life in a respectful manner for the source material, the director has to be able to film and capture the screen play on camera scene for scene, the actors have to carry the movie with their actions and dialouge or the good direction and screen play could fall flat, and the technical crew has to work hard to get everything prepared and have everything set up. Everything must be good, not one or the other, everything.
In conclusion, is it easy for WB to just release a big budget piece of crap and still get a crap load of money? Hell yeah it is! Stuff like that happens everyday. I hated X3, I could barely stand Spidey 3, and I walked out on FF4: ROTSS, but they still made money. Why? becuase big budget films with all those big heroes will attract a big crowd regardless.
The concept of Superman and Batman finally together on film alone is going to put asses in seats, but I would appreciate it if when my ass is in one of those seats, that when I remove it from that very seat I can head back to my car knowing that was money well spent to see the JLA film.