Lots of problems with the Green Lantern movie, but the main one is quite simple.
Hal's big character arc in the movie is about him taking half the movie to decide whether or not he wants to be a hero and accept the cool superpowers he has been given.
Hollywood, wake up! We don't care about people struggling with becoming a super hero. Boohoo, I'm a brilliant well paid handsome man, with his pick of women and a cool job, and now I have superpowers?! Man life is hard! That is the type of silly melodrama that made 10 years of Smallville a snooze.
When your big character arc is someone deciding whether or not to do something that we already know he is going to do, it is lazy and poor writing. But worst of all, it is boring. There is no tension, no dramatic pacing, it is just a matter of whenever the script gets around to letting the character make a decision that the audience had made an hour ago.