There are many building blocks to create the foundation for Green Lantern: the life of Hal Jordan - his father, Carol, being a pilot, the world of Green Lanterns - who they are, how they got started, Oa and all the things the ring can do. It's a tall order for a movie to cover along with a compelling story in under two hours. Which is why the movie should have been longer.
The script gave us a glimpse of a good story, the story of Hal Jordan. It's not about him turning into Green Lantern because he's got the ring. It's about the stuff in him that he has to confront in order to be this Green Lantern of Sector 2814. The movie does not delve into this enough. Had it been longer, there would have been enough room to cover this.
Another area that could have been developed more was Hal Jordan's ring constructs. He's encouraged during his training to use his imagination for better constructs, to make them unique. At first and for a good portion of the movie, his constructs are bland at best. It takes until the end of the movie for Hal Jordan to make a unique construct.
Why couldn't Hal be shown working on this, trying to tap into his imagination? The movie would have been more satisfying if this was tied with the stuff in him he has to confront in order to be a better Green Lantern. In other words, why couldn't the writers have made the movie more complex?
They should have stretched this movie to 3 hours and in the process take us moviegoers on a better character-development-ride.