I don't think the origin story is essential for GL. The reason it is for this film (from what I gather) is that a big chunk of the story is his learning to deal with his ability, overcoming his fears, etc. They could have done the same with Thor--showing how he became what he was as an adult, developing the huge ego and narcissism, examining the complex relationship with his brother and father, looking at his history learning to lead and fight, etc. But the story didn't require that, so it wasn't necessary, and no one seemed to mind. If the writers of GL also wanted to just jump into a major galactic conflict that an established GL had to deal with, the audience would have been fine with it and wouldn't have minded so much IMO. They could have given more subtle references to his finding Abin Sur, training, learning to overcome fear, etc., and I think a large part of the audience would have been sophisticated enough to read between the lines (as they were required to do a bit with Thor).
