What was the problem and what they did wrong with this movie
I think since this was the foundation film they should have spent more time with Hal Jordan on Oa training to become Green Lantern.
To do so I think they should elminated the Hector Hammond character. This would have freed up screen time not having to develop that character and how he was connected to Parallax. As for Parallax, they should have had him speaking to Jordan prior to their engagement by the DNA traces left on Abin Sur's ring.
The biggest problem to overcome making this movie is the very nature of the Green Lantern powers themsleves. The way Lantern's powers manifest themselves is very comic-y. You will something into existence. That works when being drawn in comic books and animated in cartoons. But in a live action film they don't come across nearly as well when being rendered in CGI special effects. They come across sort of hokey.
Not every comic translates well to live action.