Especially if they tie it in to a bigger DC Comics movie universe like the Avengers build up. That's what they should have done for the first one. They wanted Green Lantern to be their Iron Man, but they didn't have that tease like Iron Man that it was part of something bigger to draw in word of mouth. Even people who don't know anything about Green Lantern know Superman, Batman and probably have a vague idea of the Justice League -- even if it's just from Superfriends -- to build word of mouth. The Sinestro credits scene was a cool image for the fans, but it was never going to draw in average movie goers.
The movie still had problems, but if WB/DC had a coherent plan for what they were doing with Justice League years ago then it would have helped the box office and created more anticipation for the other movies. WB has just been behind the curve for a long time and now they are going to cobble together a Justice League movie and hope that they can piggyback Avengers when they could have made so much more money if they'd done it right.