Alright, just watched it.
I have mixed feelings about this movie. While I thought it was an alright movie, it doesn't strike as being a Green Lantern movie at all. Felt more like Star Wars. Also, felt like Sinestro was the main character. Here's some things I felt were missing:
1: Anything about Hal Jordan, why he should be a Green Lantern (nothing about overcoming great fear, Sinestro had more lines, etc)
2: Power batteries. They didn't have or use them
3: Oaths. They say it at the end of the movie, but it seemed more like a motto then. Their rings seemed to drain a few times but the broken core recharged them. They never charged themselves
4: Training on OA. How would Hal know how to use the ring already?
5: Establish Hal and Sinestro's relationship. They didn't have one in the movie. Boodika had one with him (wtf?)
6: Sequel-like ending. The others did it.
One of the main things I felt was wrong about the movie is (besides it not being based on anything but random things from GL) that it didn't give off the feeling, to me, that there is more to explore (or that I'd want to) from reading the comics. Quite honestly, the reason I started reading GL was because I really liked Hal's character in New Frontier. Prior to that, I hated the Green Lanterns (since they come off as pricks in everyone else's books). When watching this (and seeing half the corps massacred by one guy who just got his new power), it didn't really make the GLs out to be that great of characters (since Sinestro was the only one with a personality albeit one slightly different than his current comic counterpart).
If I were in charge of this movie, I would have to make it a little closer to secret origin (except I probably would take out the BN stuff). I would have it so that AT LEAST 30 minutes of the movie was pure Hal Jordan (instead of 3). I would have Hal sent to Oa for training and be partnered with Sinestro. Now here's where I take it in a different direction. I would make an original story here where the have a mission that leads into a bigger epic while underlying both Hal's and Sinestro's disregard for authority. I would leave only SMALL underlying hints about Sinestro's future and have the end (after their epic adventure) be Sinestro telling Hal that he would like to show him his homeworld someday. As the movie ends and Hal is going home, we see Korugar and can tell that it has a dictatorship going on and end the movie with Hal thinking about visiting there.
This would get us to have a sequel that is going somewhere, establish Hal and Sinestro's relationship, and have it so that Sinestro isn't just turning on the guardians out of nowhere.
Also, I wouldn't take any major liberties with the character designs ahem so that people can actually tell what characters they are looking at no matter if they watched the movie or read the comics. Also, this could work for a TV series have it be 13 episodes for the first season.
The music in this movie was really good.
Oh, and one more thing, for Sinestro's voice I would use the guy who did Andrew Ryan from Bioshock because he looks like and strongly reminds me of Sinestro.