The plot is simple: Alien gives magical ring to brave test pilot which makes him a member of the space police, and unsurprisingly a big monster is coming to destroy the earth.
I feel like they cast Reynolds wanting him to play Kyle, but the executives had decided that they were going to cast Hal because they all remembered Superfriends. Reynolds is a charming actor with a gross streak, but the movie barely lets him run loose. Peter Sarsgaard is pretty awesome, but the whole daddy-issues thing is so belabored by the end of the movie that you just don't care. Everyone else is completely forgettable (Sinestro), underutilized (Kilowog), or just flat-out boring (Carol).
The special effects are ok. Not great, but not bad either. It doesn't help matters that the whole green lantern ring power is pretty silly. Using the power of will to create giant punching gloves and green gatling guns and springs is pretty cheesy stuff. Of course, that's the bread and butter of Green Lantern: using creative, imaginative solutions to fight monsters. Fun visual gags. The movie shies away from all that, instead just letting GL do flips and float around in a green ball except for occasional moments.
What it all comes down to for me is that the movie failed to embrace the raw 'Fun' in the same way that, lets say, Thor did. Let's face it: both super heroes have a lot of silly in them. OA and Asgard are over-the-top locations. Fighting with a magic ring that can create giant fists to punch people, or using a giant hammer, are sillier weapons than a utility belt or super strength.
The difference is that Thor made fun of it, goofing on the hammer, creating a charming supporting cast of superheroes and humans that made it clear you were supposed to smile and have some fun. But GL spends huge blocks of the movie trying to make you feel like OA is Awesome and that the Lantern Corps are a big deal. Unfortunately, it just doesn't succeed; it comes off as unintentionally cheesy. It spends so much time trying to convince you it's the epic start of a massive franchise that it forgets to have the fun that you want. For example: the joy when Peter Parker first figures out how to web-sling; the thrill of a Mutant displaying newly discovered powers; or just the joy of human flight. Reynolds could have done great stuff here, but it's limited to just a few moments sandwiched between so much grandiose plotting. Ugh.
X-Men: First Class is probably the best comic book movie so far this summer. But Thor is just more straight up "fun." Green Lantern just tries so hard that it feels boring; you'd be better off seeing Super 8, which at least has fun. But there's still Captain America around the corner, and it has had the strongest trailer so far. Here's hoping!