A few thoughts (from a guy who never, and I mean NEVER, picked up a comic book even as a kid):
1. I watched Justice League as a kid. Everyone around my age did. All of us are familiar with the characters and many of us have kids now. There's a huge built-in potential fanbase. That means easy money if they don't screw it to hell.
2. I do think the Marvel heroes are much better and more interesting than the DC ones, but the DC ones aren't that bad. The Green Lantern movie was totally mediocre, but it's not a very bad concept/character. Batman is a top 3 superhero character, easy, and probably #1 in the books of many.
is stupid, but whatever. Wonder Woman has great tits. Err...wait, I mean she is a character that EVERYONE is familiar with (moreso than Black Widow) and they can rebuild that character any way they want for the movie. Flash is fun. Everyone knows him and he's just a pretty cool character. Aquaman -- well, what can you do?
I think they can do it, and do it right. I also think there's a pretty massive probability that they will do it wrong and it will suck. It's up to them.