Remember also that much of the Marvel movies' success was due to them taking characters that few people knew, and introducing them through the movies. Lots here have said that they didn't know Hawkeye, Black Widow, Thor, or even Iron Man until the movies came out. DC could, conceivably, introduce us to the likes of Martian Manhunter, Green Arrow, Zatanna, or even - since people know their images much more than they know their actual characters - Flash and Wonder Woman. They have done it before, and recently, with Watchmen.
Their biggest issue would be seeming like Avengers knock-offs now. Both companies have consistently tried to not seem like they're copying the other, dating back to the 60's, because they know it would be the quickest way for them to be seen as the second tier. Personally, I think DC's weaker characters make them second tier anyway, but there are plenty of others who feel the opposite.
Even if DC does try a JLA movie - which I don't they will any time soon - they can't possibly do it in the same way as Marvel just did with the Avengers. They just rolled the dice with their whole company by restarting the entire storyline; I think their energy will be in that, and maybe animated TV series bases on it, for a while.