I don't think it was really his fault. He's shown he can be great at serious roles. And I think RR made Hal a little less boring than he can be in the comics. I love the GL mythology around Hal, but Hal himself can be a little boring sometimes. Not as much these days though.

and Mark Strong as Sinestro is the perfect movie version. I don't see how anyone can really top it. (He needed more screen time!)
That said, I still don't understand all the hate it gets (I mean comparing it to Batman & Robin type hate.) It was decent film. Just not epic like it should have been.