Okay. As the most experienced movie goer & the most experienced GL fan on RPM, I can almost flawlessly explain everything that's wrong with this movie, as well a way they could've done it right that's SO EASY it's downright frightening that they didn't do it.
First off, I'm gonna get this out of the way right now. The CG is not the problem with the movie. In fact, not only is the CG pretty decent for the movie, but it has to be fully CG due to the character in question. Green Lantern instantly creates hard light constructs, which cannot work in a live action movie without CG. The issue with the Guardians is also not a big deal. It's an cosmetic choice, and them being real or CG takes nothing away from the movie. The CG works fine, as does his suit. When in action, the suit looks very good on him and it's a decent cosmetic choice especially considering how well it looks on other Lanterns. The constant power behind the GL logo looks awesome. I mean, to each their own regarding nitpicking the CG, but it's absolutely not the problem. The problem is that the story is jumbled, poorly told, and trying to tie up way too much into a 2 hour movie in such a way that you wonder if there isn't anything actually cut out of this movie that we'll be seeing in a Director's Cut of some kind. Here, some flaws & spoilers
[spoiler]-It starts off in the beginning with the origin of the Corps being told to us via pointless narration. Right there, it's a flaw. This is information you learn along with via your main protagonist, Hal Jordan. There are numerous people who could've told him this. Sinestro, Kilowog, the Guardians, Tomar Re, and so on. Protagonist are there so that people have someone they can relate too in terms of both wanting them to succeed as well as being brought from naivety to knowledge. To just outright explain everything up front like that is poor storytelling because by the time you're getting into the movie, you've most likely forgotten what they said.
-When trying to introduce an audience to a character which many have absolutely no [tornado fang]ing clue who he is, unlike Batman, Superman, or Spider-Man, go with a simple story. For example, Star Trek. Star Trek was a pretty simplistic story that was built around the character driven story arc of Kirk and Spock, characters that many a nerd know about but regular movie goers know little to nothing about. Star Trek not only worked, but worked FLAWLESSLY. So the idea of them using Parallax as the main villain in this movie is downright stupid. Fans know what he's all about. Average movie goer does not, nor do they care.
-There are so many goddamn plotholes in this movie. Abin Sur imprisoning Parallax. How? This needs to be explained. Hal's training, or lack of training. He literally quits and is allowed to keep the ring, as well as continue using it on Earth. That makes no sense whatsoever. If you quit, you lose the ring. Sector 2814 needs protection, so him quitting and being allowed to keep the ring is [tornado fang]ing stupid. The Guardians makes no sense and are not utilized well at all. One of them tries to harness fear, fails, falls to it, then they all try it again and form a ring, because Sinestro tells them too. Mind you, that could've worked if you actually built up Sinestro's character.
-Characters. Honestly, the actors are fine. Ryan Reynolds, at moments, shines as Hal Jordan in terms of his bravado. Blake is great as Carol and much better than I thought she would be. Mark Strong is perfect as Sinestro. Peter made a good Hector Hammond, and so on. The casting is pretty good. The problem is that you don't care about these characters because either some are not built up enough or some are built up SO much that it just becomes boring and overdone. [/spoiler]
I could honestly go into more, but I think I've made my point with this. It's just really a jumbled mess of a story complete with plot holes, a plot in which the average fan will have little to no clue what's going on, and characters that you don't care about because they're not built up enough or because you're bored. At times, the movie is fun and shows what it could've been. In fact, I'll tell you EXACTLY what it could've been and should've been. No spoilers needed...
I spent the weekend mulling over this movie, and then I decided to go back and re-read GL: Secret Origin. Secret Origin works so [tornado fang]ing well as a movie that it's scary that they didn't use it. The only thing they really need to cut out of the movie are the Atrocitus & William Hand part. Otherwise, it works perfectly. Here's why...
-Hal Jordan is a 09' Capt. Kirk/Han Solo type character. He's cocky, confident, fearless, a ladies man, irresponsible and is a perfect protagonist. He's dealing with issues both romantic, business/air force, and family, he's somewhat down on his luck, and out of the blue he's handed the most powerful weapon in the universe. Through this ring, he evolves from a character you can somewhat relate to, even though you might not like him at first, into a character you grow to love and would stand behind. Through him, NOT VOICE OVER EXPOSITION, you learn about the history of the Green Lantern Corps, explained through an awesome training montage (which both movie goers and 80's nerds love) with Kilowog, with Tomar Re. In the end, enough of his issues are resolved through the ring, both in terms of the ring helping him overcome his personal issues and the ring making him more responsible, that makes it a fantastic character arc.
-Carol Ferris is your relationship issue character. She's confident, a good business woman, responsible (a perfect counter to Hal Jordan), but also has so much weight on her shoulders from running a fledging, down on it's luck Ferris Aircraft to her relationship with Hal that works to driving their relationship closer together, especially if you involve the issues with her father & Hal and her father. Again, all of these issues of responsibility that the ring help Hal resolve with himself also help Carol out as well and thus helps to bring them closer together. In other words, hero gets the girl and everyone feels good. Yay.
-Hector Hammond is your essential PERFECT 1st movie villain. It should definitely be him like it is somewhat in Secret Origin. If you want to make him more tragic and less of a dick like he was in Secret Origin for movie purposes, that can easily be accomplished just like it was in the movie. He's powerful enough to give Hal some contention, in terms of just being a psychic beast. Plus you can up his powers for movie purposes if need be. His growing giant head helps show the audience what kind of monster he is on the inside and you start to root against him. Plus, he pines for Carol and envies Hal, so you have a relationship issue character in distress.
-Sinestro is where the movie really shines. He is your 2nd most important character and basically becomes what will define not only the movie, but the next movie: Hal & Sinestro's friendship. Sinestro is cocky, arrogant, fearless, and is essentially a more smug Hal, except much more experienced and a reason to be cocky because he is considered the greatest Green Lantern of all. He starts off having an awkward relationship with Hal, being that his friend and mentor was just killed and he does not think much of him. However, through training Hal and helping him out, he begins to see his potential. Again, a great place for a protagonist to learn more stuff about GL lore that the audience can learn along with him. At the end, after the conflict between Hal & Sinestro vs. Hector Hammond, with Hal being the one who earns victory, Sinestro and Hal's friendship becomes clearly apparent, Sinestro starts showing Hal some respect and will stick up for him against the Guardians (if you want to throw that in there), and it perfectly sets up for the next movie when Sinestro tells Hal "When next we meet, I'll show you MY sector!" which of course sets up for the fall of Sinestro, which average movie goers won't see coming and which GL fans will be geeked out for. (Hell, you can even include an after credits spoiler in which Sinestro returns to Korugar which has the GL symbol everywhere and that has pictures and statues of Sinestro all around). It can be done numerous ways.
Essentially, as shown with Star Trek 09' and which what should've been done with the Star Wars prequels, you make the GL movie a character story arc between Hal & Sinestro, as well as throw in the relationship with Hal & Carol. This way, you connect with the audience and the story doesn't matter as much because you don't care as much about the story as you do about the characters. A movie based on Secret Origin would do that so well. This movie failed at that.
I trust I've made my point clear.