IMHO, there were lots of things wrong with it. Not read the comics until after seeing the movie, mind.
You have your stock angsty hero, because every superhero movie needs one. In this case, he's angsty because he's got a great job, well paid, can be as irresponsible as he likes and everyone still loves him. Um...? As well as that, there was far too much angst. The main character just wasn't interesting at all.
The pacing was all over the place, you had lots of scenes that went nowhere at all, and big slabs of blather not broken up with anything.
The most interesting thing in the movie were the other lanterns, which is why they were in the ad. In the movie you get one panning shot of them in the middle, and one at the end, lasting seconds in each case and about 5 of them actually do anything. Had some good bits where he fights Sinestro and Killowog, and that lasts only a few minutes.
The 3D elements, like most 3D stuff, was stuck on as an afterthouht, a few bits almost at random made 3D, the rest left alone.