#45 Throw It into the Sun
Parallax is defeated by being thrown into the sun. Remember when everyone complained about Superman doing that in Superman Returns? This is the same thing. Its not a satisfying resolution at all.
#46 The Climax is Over Too Quickly
Hal uses a handful of constructs on Earth and a couple when the fight moves to space. He gets knocked into a couple of cars and gets a love tap to the shoulder; theres no sense that this is a really hard battle. This is most likely because its almost impossible to make fight a giant cloud exciting.
#47 Thanks, Guys
Sinestro, Kilowog and Tomar Re save Hal from falling into the sun after throwing Parallax in, so they were just watching the whole time without helping.
#48 Gravity Does Not Work That Way
As soon as Parallax goes into the sun the gravity seems to have been shut off and all four Lanterns float there comfortably.
#49 Crappy Dialogue
Carols second to last line in the film is Thats really cool. Great writing.
#50 Sinestros Corruption
After the first bit of credits, we see Sinestro put on the yellow ring for no reason. Was he consumed by the power of fear? Did he actively decide to put it on or could he not help himself? Its unclear because it comes out of nowhere and nothing about the story makes it seem like a reasonable outcome. It was shoehorned in just for fanboy recognition. Need I go on about how the people who made this movie were afraid of the backlash of fanboys?
Bonus Reason: No Alan Scott
Alan Scott was the original Green Lantern created in the 1940s. His powers came from a magc talking green lamp, a piece of which was carved into a ring. There were no Guardians or other Green Lanterns wandering around; just him. He was a founding member of the Justice Society of America (the precursor to the Justice League) and just generally awesome, but his comics were eventually cancelled after World War II. In the late 50s, DC Comics revamped the idea with Hal Jordan, making him a space policeman instead of a vigilante. Alan Scott was a much better character than Hal; he was a lot tougher and generally hit his head a lot less often. The irony of the failure of the Green Lantern movie is that it was all caused by fear, fear from the film makers whose aversion to risk was obvious. Green Lantern was defeated by fear; even though it's sad, you have to admit it's at least fitting.