The basic origin story was fairly faithful to the comic book but did take some slight deviations. Also, with DC it's hard to know what is "faithful" to the comic anymore. Green Lantern has probably had three major tellings of his origins over the 50 or so years he's been around. This was probably pretty faithful to the most recent one. That aside, the story suffered from forcing the themes down the viewers throats rather than letting them see them displayed through the actions of the characters.
It also read as DC's attempt to make a Marvel movie. That's an understandable thing right now given that Marvel movies are rocking the super hero movie world right now. But DC and Marvel characters are different from each other. Marvel origins are very much about redemption and humanity. The characters in Marvel start at a low point and must take on their heroism to redeem themselves. This is not the template for a DC character. Most DC characters are naturally good people who are granted powers for who they are already rather than what they can become. I found it very hard to believe that the ring chose this guy out of the 7 billion people in the world.
I did find Sinestro to be an interesting character and wished there had been more interaction between him and Hal Jordan. This was a prominent part of one of the recent DC Universe animated movies and makes the eventual turn of Sinestro mean so much more. It would be akin to not giving Batman much time with Harvey Dent before his turn to Two-Face.
The biggest issue with this movie was casting. I don't mind Ryan Reynolds, but Hal Jordan he is not. I could see him as a wise-cracking Wally West or he was okay as Deadpool, but something about him seemed off as Hal Jordan. He was too aloof and really a big jerk. He's not the only casting problem. Blake Lively really doesn't live up to her name with her cardboard acting and lack of chemistry with the lead. I didn't buy her as an aspiring entrepreneur nor as a friend of Hal Jordan's.
The effects weren't bad, but I do question the need to change the classic costume from the comic book version. The Gil Kane-designed outfit is one of the greatest costumes in comics. It's a shame that we still have not seen it truly realized on screen.