The Humour---
Even if the plot of this movie had issues, this movie should have at least been funny with Ryan Reynolds as the main lead. The jokes in this movie either fell flat or I already saw them in previews. Honestly, the funniest part of the movie was Hector Hammond. I literally laughed almost every time he opened his mouth. The only thing is I don't know if they were intentionally trying to make him that ridiculous.
Some people had slight issues with the plot of Iron Man 2 but the one thing most everybody can agree on was the movie was funny and charming. Same goes with Thor and even Incredible Hulk to a certain degree. Green Lantern obviously tried to go the Iron Man route and it didn't work as well. Green Lantern seemed kinda stuck between the uber-sterile Superman Returns and the humour of Iron Man. I think if they do the sequel, they should fix this.
The Score
Easily one of the most forgettable musical scores for a major superhero movie. For a sci-fi/superhero movie---I was expecting something grand. Instead, it sounded like they got their music from the factory where they get scores for crappy 'epic' movies. It wasn't horrible but it wasn't memorable. I was honestly hoping for something like the Star Trek 09' soundtrack or Iron Man 1.
The Story
Less Earth and Blake Lively. More space and Oa. All that needs to be said. The movie really shined near the end when earth was in trouble and GL fought Parallax by himself. The other GLC had nothing to do in the movie and were somewhat wasted as supporting characters. Less Hector Hammond and his daddy/Carol Ferris issues. Kind of felt out of place to me with the greater Parallax threat. Also, I felt the Green Lantern Corps were d-bags. They didn't even help Earth and weren't very heroic at all---they may as well have been villains.
The CGI/Sets/etc.
I thought they used too much CGI. The beginning of the movie honestly felt like a videogame cutscene. As did parts on Oa. It's kind of odd when RR's head is the only real thing in the scene. The suit looked cool during some parts of the movie and pretty silly in others. The constructs looked cool though.