The basis for Green Lantern is this:
On the distant planet Oa, the guardians of the universe created the Green Lantern Corps, a group made up of 3600 individuals recruited from across the universe. Using their green power rings—technological weapons of incredible strength, fueled by their user's willpower and imagination—they act as intergalactic policemen, keeping innocents safe from various threats.
Hal Jordan, an overly cocky fighter pilot, has now been recruited as the first-ever human Green Lantern. Meanwhile, Parallax, a massively powerful and evil being made of pure fear, is on a course headed for Earth. Hal must deal with his own personal fears before being able to successfully wield the ring's power and defeat Parallax.
The bizarre thing about this film is that it gets so much of the Green Lantern universe right, but doesn't make it interesting. Hal Jordan's story is somehow robbed of its emotional resonance from the comics, despite the fact that most of the story is exactly the same. It's the little moments that are missing; the ones that show Hal as a compelling character. The same goes for every other character in the film. The audience is simply never given a reason to care about any of the main characters.
The best scenes on an emotional level are the romantic bits between Hal and his former girlfriend, Carol Ferris. While Carol is perhaps shown too often as nothing more than a support for Hal, her scenes are genuinely sweet. One moment in particular is notably hilarious, and contains the best genuine laugh in the entire movie.
The script in this movie is awful. Every single line is terribly bland. Hal is somewhat funny, but it seems as though most of that humor was due to Ryan Reynolds' improv skills and comedic timing. The overall story and editing are terribly problematic as well. The film jumps back and forth between plotlines in ways that are more frustrating and distracting than anything else. Furthermore, the interesting plotlines (those involving the Corps) end up going nowhere. Certain plot twists come up only in order to set up a sequel, and the main villain isn't even a real character.
Parallax isn't a character; he's just a giant puffy-headed death cloud. The only character that's actually developed as a villain is Hector Hammond, but he's more of an annoyance in the film than anything else. He's not interesting, he's not fun to watch, and he only distracts from Hal's story.
Still, however, there are enjoyable things about the movie. Seeing Hal Jordan use his power ring in superheroic ways inspires a bit of childlike glee. The action, for what little is there, is fun. It's that tiny bit of enjoyability that saves Green Lantern from being terrible. And, to be fair, there's nothing in Green Lantern that's particularly offensive; it's just extremely bland.
The eight-year-old in me wants to give this movie a six out of ten. The twenty-one-year-old in me wants to give it a four. I'll split the difference. Let's just hope that if and when a sequel is made, it's better than this one.